Unwanted lines appearing around pictures

[Leen Lauriks]

A while after inserting a picture in my word-document, a line appears around
this picture (most of the times only above the picture), which is not in the
original picture file. It's not a border and it can only be deleted by
cropping the picture (but then I loose some pixels of the picture itself too
and the line reappears again after some time).
Anybody else already had this problem? Suggestions to solve this?

Thanks,
Leen