One thing that caught me a few times before I figured it out - APUG will accept images sized to 850x750, but not 750x850. In other words, images that are in portrait orientation may have a maximum long dimension up to just 750 pixels, while images in landscape orientation may have a maximum long dimension up to 850 pixels.
The maximum file size for APUG galleries is 512KB. The acceptable image formats are jpg, jpeg, gif and png.
Here is a link to the allowable extensions and sizes:
For APUG, scanning from slides is fine. For APUG, scanning from prints is preferable, but scanning from negatives is acceptable.