Q: Why is Sku Fetch inserting Images inside item descriptions?
A: The problem is that your supplier is putting very big images in the description of their listings. Your listing template is one that is formatted into columns, and those pictures are wider than those columns. This is making your listing span over to the right and off of the page.
The best recommendation would be to have sku fetch remove supplier hosted images from the description as you list them. There is an option for this in your Filters and Formatting Settings in Sku Fetch called Auto remove Supplier Hosted Images from Description. 
Please turn this option on and that will help you with future listings. For the existing items, the only thing you can do is edit them and try to remove those large supplier images from your description.