I checked the insert image function and I know that is working properly. Some questions:
1) Which function are you using (replace, insert, or delete)
2) What rank of the image are you doing. Rank is simply the image order of that listing with 1 being the main image. Depending upon which one your deleting, the order may change.
I will check what you are doing on my end with the info you send. Normally it will give an error if it doesn't work, but something else may be going on.