Unfortunately, none of these suggestions fixed my problem. My original graphics were screenshots I took and saved as jpgs. I added these to a layout, exported to pdf and the graphics were missing from the pdf. (I should mention that I've done this many times before and the export to pdf worked fine).
I tried converting the jpgs to different image formats, saving the graphics in the document, changing export settings, reconstructing the mxd and nothing solved the issue.
I also discovered that when I opened other similar mxds (containing a layout with added graphic files) that I had successfully exported to pdf in the past, they also dropped graphics when I tried to export to pdf now.
I never found a way to get the pdf exported from ArcMap successfully. I was sending something to a printer and ended up delivering the pdf file with the missing graphcs. I also gave the printer the 2 jpg files and they added them back in to the pdf before printing.
Very frustrating - especially when I discovered that layouts that had exported successfully in the past no longer export to pdf successfully now.