In the case some from the images in the image library is missing (in the case of previous failure the image has been deleted deleted or something else happened the image file just does not exist), then the upgrade fails and the database remains inconsistent, so the upgrade cannot be executed.
The upgrade should be made so it does not fail completely, but just should not take over the concrete image or delete it or, replace with some standard "failed upgrade" picture with the same file name, log this event into the log and continue. Or, at least, if it fails from that reason, it should write down the list of all pictures missing, otherwise the recostruction of the image folder and file structure can become very very problematic - to figure out one missing picture one new run of upgrade is necessary, then again the DB restore, etc.
Following errors are in the log in the case of such problem:

14:41:57,921 INFO [DialectDetector:80] Determine dialect for PostgreSQL 9
14:41:57,968 INFO [DialectDetector:145] Found dialect org.hibernate.dialect.PostgreSQLDialect
14:42:29,640 INFO [BaseDB:406] Database supports case sensitive queries
14:42:29,890 INFO [UpgradeProcess:205] Upgrading com.liferay.portal.upgrade.UpgradeProcess_6_1_0
14:42:29,890 INFO [UpgradeProcess:205] Upgrading com.liferay.portal.upgrade.v6_1_0.UpgradeSchema
14:42:33,000 INFO [UpgradeProcess:205] Upgrading com.liferay.portal.upgrade.v6_1_0.UpgradeUserName
14:42:33,203 INFO [UpgradeProcess:205] Upgrading com.liferay.portal.upgrade.v6_1_0.UpgradeAdminPortlets
14:42:33,390 INFO [UpgradeProcess:205] Upgrading com.liferay.portal.upgrade.v6_1_0.UpgradeBlogs
14:42:33,718 INFO [UpgradeProcess:205] Upgrading com.liferay.portal.upgrade.v6_1_0.UpgradeCamelCasePortletPreferences
14:42:35,546 INFO [UpgradeProcess:205] Upgrading com.liferay.portal.upgrade.v6_1_0.UpgradeCountry
14:42:35,750 INFO [UpgradeProcess:205] Upgrading com.liferay.portal.upgrade.v6_1_0.UpgradeDocumentLibrary
14:42:36,031 INFO [UpgradeProcess:205] Upgrading com.liferay.portal.upgrade.v6_1_0.UpgradeExpando
14:42:36,046 INFO [UpgradeProcess:205] Upgrading com.liferay.portal.upgrade.v6_1_0.UpgradeGroup
14:42:36,406 INFO [UpgradeProcess:205] Upgrading com.liferay.portal.upgrade.v6_1_0.UpgradeImageGallery
14:42:37,312 ERROR [MainServlet:198] com.liferay.portal.kernel.events.ActionException: com.liferay.portal.kernel.upgrade.UpgradeException: com.liferay.portal.kernel.upgrade.UpgradeException: com.liferay.portlet.documentlibrary.NoSuchFileException: C:\DEVELOP\Liferay_6.1.0\bin\data\images\10\42\10423.png
com.liferay.portal.kernel.events.ActionException: com.liferay.portal.kernel.upgrade.UpgradeException: com.liferay.portal.kernel.upgrade.UpgradeException: com.liferay.portlet.documentlibrary.NoSuchFileException: C:\DEVELOP\Liferay_6.1.0\bin\data\images\10\42\10423.png
at com.liferay.portal.events.StartupAction.run(StartupAction.java:58)
at com.liferay.portal.servlet.MainServlet.processStartupEvents(MainServlet.java:1267)
at com.liferay.portal.servlet.MainServlet.init(MainServlet.java:195)
at javax.servlet.GenericServlet.init(GenericServlet.java:160)
at org.apache.catalina.core.StandardWrapper.initServlet(StandardWrapper.java:1266)
at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:1185)
at org.apache.catalina.core.StandardWrapper.load(StandardWrapper.java:1080)
at org.apache.catalina.core.StandardContext.loadOnStartup(StandardContext.java:5001)
at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5289)
at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:866)
at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:842)
at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:615)
at org.apache.catalina.startup.HostConfig.deployDescriptor(HostConfig.java:649)
at org.apache.catalina.startup.HostConfig$DeployDescriptor.run(HostConfig.java:1581)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:619)
Caused by: com.liferay.portal.kernel.upgrade.UpgradeException: com.liferay.portal.kernel.upgrade.UpgradeException: com.liferay.portlet.documentlibrary.NoSuchFileException: C:\DEVELOP\Liferay_6.1.0\bin\data\images\10\42\10423.png
at com.liferay.portal.kernel.upgrade.UpgradeProcess.upgrade(UpgradeProcess.java:211)
at com.liferay.portal.upgrade.UpgradeProcessUtil._upgradeProcess(UpgradeProcessUtil.java:93)
at com.liferay.portal.upgrade.UpgradeProcessUtil.upgradeProcess(UpgradeProcessUtil.java:46)
at com.liferay.portal.events.StartupHelper.upgradeProcess(StartupHelper.java:81)
at com.liferay.portal.events.StartupHelperUtil.upgradeProcess(StartupHelperUtil.java:40)
at com.liferay.portal.tools.DBUpgrader.upgrade(DBUpgrader.java:119)
at com.liferay.portal.events.StartupAction.doRun(StartupAction.java:144)
at com.liferay.portal.events.StartupAction.run(StartupAction.java:52)
... 20 more
Caused by: com.liferay.portal.kernel.upgrade.UpgradeException: com.liferay.portlet.documentlibrary.NoSuchFileException: C:\DEVELOP\Liferay_6.1.0\bin\data\images\10\42\10423.png
at com.liferay.portal.kernel.upgrade.UpgradeProcess.upgrade(UpgradeProcess.java:211)
at com.liferay.portal.kernel.upgrade.UpgradeProcess.upgrade(UpgradeProcess.java:225)
at com.liferay.portal.upgrade.UpgradeProcess_6_1_0.doUpgrade(UpgradeProcess_6_1_0.java:67)
at com.liferay.portal.kernel.upgrade.UpgradeProcess.upgrade(UpgradeProcess.java:208)
... 27 more
Caused by: com.liferay.portlet.documentlibrary.NoSuchFileException: C:\DEVELOP\Liferay_6.1.0\bin\data\images\10\42\10423.png
at com.liferay.portal.image.FileSystemHook.getImageAsStream(FileSystemHook.java:74)
at com.liferay.portal.upgrade.v6_1_0.UpgradeImageGallery.migrateImage(UpgradeImageGallery.java:357)
at com.liferay.portal.upgrade.v6_1_0.UpgradeImageGallery.migrateImageFiles(UpgradeImageGallery.java:398)
at com.liferay.portal.upgrade.v6_1_0.UpgradeImageGallery.doUpgrade(UpgradeImageGallery.java:248)
at com.liferay.portal.kernel.upgrade.UpgradeProcess.upgrade(UpgradeProcess.java:208)
... 30 more