images hide table borders in .pdf

[eNGiNe]

A colleague has prepared a .docx where a table includes two small graphics. When he's looking at the content in Word, or when we print direct to a physical printer, no problems; when he tries (various) output to .pdf options, the background for the graphics is on top of (breaks) the table borders.

Any tips on a reliable procedure to avoid this? He'd prefer to distribute the content as .pdf.