1024x768 is the largest size that PowerGallery currently uses, so I would say that should be your minimum size. All of the sizes are created automatically from your source image to work with the galleries.
In terms of dpi and bit-depth, I don't think it matters too much since the images will be auto resized. If possible i would convert a few of your images and upload them just to make sure the results look good to you.
-justin