Hi Joan - I think Donna has the right idea about taking your potential buyers and what you are selling into consideration, as well as dial up customers. That aside, I personally like to have the supersized version of my photo(s) be large enough to be visually impressive, and to show a lot of sharp detail. This means a well done photo ie. sharp, in-focus, well lit, close-ups as necessary, proper backround, etc. The thumbnail in your listing will be downsized automatically to 400x300. That means your uploaded pic should be LARGER than that, otherwise there is nothing to supersize too, except the same size picture.
The maximum size Auctiva will allow is 1024x768. If you try uploading a bigger image, it will get downsized to that. I like using 1024x768, so my supersized pic is big, but that's just me. 800x600 might be a good compromise. Some folks don't like the supersize feature at all and will just stay with 400x300. It's all a matter of preference, taking factors such as Donna mentioned into account.
The main thing is to have GOOD photos. Just look at all the crappy photos on ebay, I mean REALLY BAD photos; it seems many sellers have no idea how to take a decent picture, or why they even need a good picture. They just wonder why their item won't sell.