By design, if you customize one of the existing Auctiva templates, the images included in a listing using that template will supersize. You would need to get into the HTML and CSS of the template in the template editor to remove the supersize ability, but unless you are savvy with HTML and CSS it would be very difficult to then use the Image Selector on the lister to get your images to display appropriately with your selected layout in the template when you are ready to post as the templates are designed to have the image enlarge when they are click upon.
However, you could customize one of our existing templates as you wish and select 0 images to be added to the template and then, when creating your listings, use the Insert an Auctiva Hosted Image tool that is available in the Description Editor on the lister to add your images directly to the listings in your description. The button for this tool is available in the lower of the rows of tools in the editor just beneath the Bold button and to the right of the insert a link tool.
This tool will allow you to place your images directly into your description either at full size, or, once added, you have the ability to alter the display size of the images. Once they are in your description you can right click on the image to access the image properties and change the size to suit your needs.
Should you run into any difficulty with this, please feel free to file a support case from under the Help tab of the site letting us know where you're encountering trouble and provide an example listing and our support team will be glad to help.