Step to reproduce: Insert an image in the text. Select it Click on right button Select "Compress image" The dialog pops, but the help button does not bring a help page.
Olivier Hallot committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/help/commit/?id=e22b2bc0beeb1bb32184af509af98c303677032c tdf#95490 Help page for image compression
It must be noted that the help patch supplies the target help page but does not fix the help button issue of the dialog.