Step to reproduce:
Insert an image in the text.
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":
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.