Hi, just in case someone else has the problem.
Under QGIS and gnome I couldn’t print images from the print composer.
Apparently, it was due to
qgnomeplatform 0.7.0-1. It seems is has been solved with 0.7.0-2 but it is only available in AUR (not in stable, testing or unstable yet).
I didn’t have this issue on another computer (also manjaro gnome stable)…
qgnomeplatform had been brought on my laptop by
manjaro-gnome-assets. I removed it and all its dependencies and the problem is solved.