Hi Tony and Subsuper!
Thank you both for your help.
In future I'll compress my jpegs to "medium" (quality 5) before I upload. No noticeable loss of quality and I save disk space.
I'm puzzled about the visible difference in the image appearance when I look at the image embedded in Auctiva’s supersized image web page (viewed with Microsoft Explorer or with Google Chrome) contra the same image as the jpeg before uploading (viewed in Photoshop or with Windows Live Photo Gallery). The image is noticeably more vibrant in the Internet browser, particularly the orange colors. I'm at a loss for an explanation!
I deliberately keep to 800 x 600 pixels rather than using Auctiva's maximum of 1280 x 1024 pixels - for the type of things I sell, that's more than enough supersizing, and when the buyer looks at the supersized picture, he/she can see the Auctiva navigation buttons and the image at the same time without needing to scroll.
A side remark: It's rather impressive how fast an Auctiva employee (here Tony) jumps into these forum threads where relevant with their help. I don't think I've ever come across a similar company with such outstanding service.
Again, thanks for all the help,
Patrick