I am having the same issue. Did you ever find a solution?
I have tried deleting and then re-importing the images, but they never end up included in the project.
When I want a new image in the help, I start by manually placing the image file in the help project's "Images" folder with Windows Explorer (where ever you normally put the pictures). Inserting images from there to help topics seems to work perfectly for me.
I erased the original images and re-inserted the new images. Now things are fine.