Image duplication when inserting new product

[danward]

I've got a very annoying problem which I am unsure how to fix. When I add a new product to my newly built shop, the new image is uploaded and displays fine, but it sometimes replaces a previous items picture with the new one as well. I have no idea why it might do this?

 

Any help appreciated, I really need to stop this happening.

 

I have installed Dynamic Image Resizing but I don't know if this would effect it?