I am having this same issue although I am not trying to create a map series so this solution does not work for me. Here is the situation. I have an image (I have a tiff, jpg, and/or png to pick from) that was created in ArcScene. I do not create the image so I do not know the details of their workflow. I bring the into my ArcGIS Pro 3.1 layout. It looks fine there. Then I export the layout as a PDF and the frame around the image appears in the PDF. I have another image (the logo) also on this layout that DOES NOT create a frame around the image. If I export to a 24-bit true color tiff (no compression and 600 dpi), I get a very nice image but when that image is saved as a PDF, it will not OCR which is important because the PDF will be published eventually. I brought the image into ArcMap and exported as a PDF and it looks fine, no frame. Another note, if I bring the PDF into Photoshop and save it back out as a Photoshop PDF, it 1) removes the frame but 2) it creates an enormous file and 3) messes up the colors...a lot. No solution there. So this seems like an ArcGIS Pro problem. Any additional help on this issue would be greatly appreciated.