Edges of images show up in PDF

I have had this problem on and off, and I cannot make sense of it. I have several GIF images that were created in Illustrator and/or Photoshop and then imported (linked) into an anchored frame. When I generate a PDF, some of these images show a light or dark grey boundary line on the right side. Resizing the images in Photoshop sometimes works, sometimes it doesn't. When regenerating the PDF with a couple of images changed, some of these are OK but others are not and sometimes images that were OK previously show the grey boundary line. Always to the right and not consistent throughout the document.

I am using High Quality print settings for the PDF. I am working in structured FM, but I guess this 'feature' has nothing to do with structured vs. unstructured. Maybe it is a bug in FM's handling of GIFs, although I do recall having similar problems with EPS files earlier. Can anyone tell me what causes the lines, and how to keep them from messing up my PDFs ?