I'm sorry to hear about the difficulty with image sizes. In the prior version of the template editor there were some options for different image tags that would allow you to use the image selector to input full sized images in your description and those tags would use images at different sizes (note that we make two different copies of your images after you upload the original, a thumbnail version and a template sized version). These tags were related to the prior templates and have been deprecated along with the older templates.
However, you can still use the tool in the Description Editor: Insert an Auctiva Hosted image to insert full size images in your item descriptions. This tool is available in the lower of the toolbars for the editor about midway in the toolbar (next to the Link button) and the images will go in your description where the cursor is in the editor when you select this option.