In 7.3.10 GA1, Guests simply cannot use the Upload or Image fields. This behavior will change for 7.4 and possibly be brought back to 7.3 in a Fix/Service Pack.
The new behavior is well documented in the summary of LPS-123369
As a form adm, I want to allow guest users to upload files when filling out an upload field
Create or update documentations related to the upload field