Lines appearing above and below graphics

Hello

I've imported an FM book into a RoboHelp 2015 project. Many of the graphics appear (in both the editing pane and after generation) with a black line above them (and sometimes below them too). These lines were not in the original.

When I open the graphic in a paint program, crop out the lines, and save the file back to the RH folder, it is not automatically refreshed. Still, when I delete the offending graphic from the topic, go to reinsert it, and browse to its folder, I can see, via its thumbnail, that the line has been removed. But when I re-insert it into the topic, the black line appears again!!!

What is going on?

Regards

Geoffrey Marnell