Weird, both the official Reddit app and Lunar for Lemmy also have a custom icon feature and they don’t seem to be having the same issue.
Even after restarting my phone in order to fix it, Voyager “forgot” its icon again after simply closing and reopening it. I did not even change the icon.
EDIT: after some experiments I found out that this problem only seems to occur when the “O.G.” is used. Is the iOS app perhaps simply missing a small version of that icon?
This isn't any different from how OG Lemmy works.
You can only have single image on the post itself, if you want to add more, you have to add them via Markdown in the body.
Unfortunately there isn't any support for gallery type posts at the moment, but feel free to badger the devs about it. Looks like there's been an open issue about this since 2020.