Limit image file size to 256kb

This limits uploaded image file sizes to 256kb, as per the openbadges spec.  As a pleasant side effect, it also resolves a database disconnect issue that was happening when someone attempted to upload an image over 2 megs.

I haven't added any additional copy to the Visuals tab stating this limit (I wasn't really sure where to put it without making it seem rather wordy/ugly), but if you attempt to upload an image that is too large, the error message "Maximum image size is 256 KB" will be displayed.