Ticket created by ~spaetz on ~martijnbraam/Megapixels
https://git.sr.ht/~martijnbraam/megapixels/tree/master/item/postprocess.sh#L69 contains if [ "$CONVERT" == "convert" ]; then but is supposed to be run by "sh".
== is a bashism according to https://mywiki.wooledge.org/Bashism (and it fails indeed on Mobian), so simply replacing s/==/=/ should fix this.
Thanks for Megapixels!