If the client can stand a short latency between upload and display:

It will take about four hours to have a graphic artist show you how to add a watermark to an image in any of the major graphics programs like Adobe Photoshop or Corel PhotoPaint, and about 10 minutes to actually do the watermarking.

Have him upload to a holding-file; write a script that alerts you by email of any upload; then download it, watermark it, re-upload it to the display file. Or do it once a day, etc.

Sell it as a way to avoid heavy up-front costs to develop software, as he will only pay a small unit fee for each image as they are produced. This may create a small continuous revenue stream for you. Some day you'll pay your 14 year old nephew 1/4 the fee to do it for you.