I've tried opening the Accessibility menu and choosing black background with green font, but it seems this does not work if the document was originally scanned in Black/White mode. Is there any other way to enhance the PDF's contrast?
I do not think that any of the PDF viewers have built in filters or adjustments for pictures, only for fonts backgrounds and colors.
You could adjust your monitor, and/or if your video card software supports it adjust the video cards, video settings. Save the video settings in a profile to use again, for this purpose, it would not be too bad.
You could print it and adjust your printer (most printer preferances allow adjusting).
See also Adjust PDF Contrast with Adobe Reader/Acrobat?
You did not indicate if it is one picture, or many, which would help the question. You did not indicate the operating system (even a tag for it works). adding that information to the question will improve being able to answer.