Hi new_seller21,
Thanks for contributing to our forum. Ebay's system should automatically resize any Gallery images you submit for your listings for optimal display in that area when you list through our service, so you should not have to observe any specific image size requirements just because you create your listings through Auctiva.
However, the following page of the eBay AU Help index indicates that each image must be larger than 440 px on the longest side and that each image should be at least 1000 px on the longest side is recommended for best results with their Supersize feature:
http://pages.ebay.com.au/help/.../photo-error-SS.html
I may be able to determine what caused this if I was able to review a couple specific examples of the behavior, but it appears that you have already corrected the issue in any of your active eBay listings that exhibited this problem.
I hope this helps. If you continue to encounter difficulties of this nature, please feel free to contact our Customer Support team by mousing over the “Help” tab within your account and selecting the “File Support Case” option.
-Mike