Hello buncoman -
When you initially open a listing using Create Similar, are you using the link to "Remove All Images" and the link to "Remove All Optional eBay Gallery Images" on the listing creation page?
If you remove all of your images from the base listing first, you should be able to add new images directly without the old images remaining associated with your listings. If you have used the tool in the description editor to add images to the body of your listing in the description text, these would need to be removed manually from that area.
If you continue to experience difficulty, you can also try removing all of the images in the base listing using the "Remove" links beneath each thumbnail on the listing creation page.
Should the difficulty persist, please provide the details of what you experience, including the steps you are taking when adding images to a listing using Create Similar in a support case so we can look into this further.