Hi,
The [IMAGEx] tag uses the stored 400 x 300 max image, so a square image stores at Auctiva at 300 x 300 (governed by height limit) for the "Standard" image. Yes, that would pixelate going higher to 350 x 350. You could drop-back to what you've been doing with your own click to supersize using the original image URL or mimic Auctiva's query code server call for the supersize page. Downsizing the image (except to micro-thumb size) generally does look fine, and I assume that's what you've been doing. Note, that query code call with a string of image numbers has possibility for the multiple image supersize you wanted. It would require some scripting to build the query string from URL substrings.
This is why (at least one reason) I'm working on new viewers to compliment the Auctiva stock methods.
Danno