If you upload an image larger than 1024x768, Auctiva's uploaders will downsize it to that maximum allowable size. I find it best to downsize first on my computer; then it will upload faster.
Downsizing first also allows your pic to have a larger file size after it is uploaded, because when Auctiva downsizes the dimensions of your pic it also compresses it to a smaller file. This can sometimes result in a pic that isn't quite as sharp and detailed as you might want, especially noticable with a larger picture like 1024x768.
If you downsize to 1024x768 or smaller first, Auctive will not compress it further. Thus you can have more control over the quality of your picture when it is supersized.
Many of the free image editors give you control over compression (quality), resizing, and many handy editing effects, more than Auctiva offers. Try several and find one you like. You have more control over your photos by editing before uploading to the site.