The red x indicates you do not have the thumbnail photos where the web page expects them to be located. It has been a while since I used PixConverter, so I don't remember for sure, but I think the photos (thumbnails) are placed in the same location as the web page. The solution to the red x will be to upload the photos.
I don't think you will be able to use the PixConverter to add anything fancy. PixConverter was designed to make a simple thumbnail page and display the full-sized photos.
If you have the code for your add to cart button, you can use an HTML editor to edit the thumbnail page or each of the individual photo pages and add that code.