The 1024 x 768 is "pixels", which dictates how it will display on a pixel based monitor. Digital cameras also operate on pixel for capture, e.g. 6.1 Megapixel translates to highest resolution for my camera.
When I crop, I'm not loosing resolution, i.e. I'm not losing density. When I resize (or let Auctiva resize), resolution is lost by forcing less detail per pixel retained.
An ungraded coin does well as square, which means the height limitation of 768 pixels is in play. I do close-ups at 6.1 MP or my scanner at 1200 DPI, crop-in to get the coin with small amount of boarder/background and upload the result, allowing Auctiva to resize. Keeping common aspect ratio or crop to specific size (like 8 x 10) can usually keep them uniform, but it also depends on how you want images displayed in a listing for best viewing of item. There's no absolutes, just best fits for the occasion.
Here's one from my photo/scan library that's stored on my PC at 2064 W x 2055 H (nearly square) and resized 771 W x 767 H for supersize by Auctiva at upload. The full-size limits are 400 W x 300 H for storage and display.
Edit: I added the thumbnail (limit 80 x 80), so you could see the set stored by Auctiva (minus store at 96 x 96).