Hi,
One method is use of the image URL tag, e.g. [IMAGE1_URL]. That pulls the original stored image and is used with your own HTML code, e.g. <img src="[IMAGE1_URL]" alt="" height="500" width="500"> If you upload a square image at the limit 768 x 768, that code will insert the image and display at 500 x 500.
You could also use the original image tag and the code builder will add the HTML code, e.g. [IMAGE1_ORG]. That tag inserts at the full size, so a resize of any larger (before or after upload) to 500 x 500 is necessary to do the same job.
You can still select the header/gallery, as always. Auctiva will send them your selected original image and eBay will downsize it for the header and gallery, as always. eBay's 500 x 500 is larger than before, but still not as large as Auctiva's 1024 x 768 limit.
Danno