idobrinescu
No need to flatpack or snap for xnviewmp, ou can use appimage version, see last version here: https://newsgroup.xnview.com/viewtopic.php?f=82&t=39939 , xnviewmp contains xnconvert (see bulk edit)
Linux x64 TGZ version also good, extract to opt/ and copy the launcher .desktop to .local/share/applications folder, adjust the icon path if you want.