I've uploaded three files. The first (input.pdf) is a small test file with
images at 300dpi, the second (output-few.pdf) was obtained by running
GhostScript on the input file with the following minimal options
PDF Checkpoint says that output-few has images at 30dpi and that output-many
has images at 72dpi. Of course, this could be a bug in PDF Checkpoint but
since we couldn't find many other checkers, we decided to be cautious. If
this checker gets it wrong (while getting others right), then perhaps other
apps in a printer's workflow will be confused too.