Adding Still Image to existing Collection produces general Error message

I am failing at this very basic task so I must be overlooking something.
Omeka Version 2.1.2
Mac OS X 10.8.5
Firefox 29.0.1

I create an item, define it as a Still Image and fill out the Item Type Metadata, partly fill out the Dublin Core fields, browse and select the image to be uploaded (JPEG, RGB, 2541 × 1692 pixels, 954 KB), add the relevant Tags, and assign it to an existing Collection. When I click Save Changes I get the nondescript “Omeka has encountered an error”. It still produces the item in the collection I assigned it to, but the item is untitled and empty (no Dublin Core data, no Item Type Metadata, no Tags, and no associated File).

What am I doing wrong?

I forgot to say that if I then try to Edit that existing untitled and empty Item, it again produces the same error message.

Did some more testing, and anything but uploading the image seems to work, so the actual problem is the image file I want to upload to the item. The file size, type nor name shouldn’t produce any problems.

Name: 100_0855_DEF.jpg
Kind: JPEG image
Size: 952.573 bytes (954 KB on disk)

If you follow these directions to turn on error logging, you should be able to retrieve a more specific message:

Are you having this problem with all images, or just this particular file?