So PureOS is not shipping the Gnome 3.38 release version of eog, which from my testing on the PinePhone scales perfectly?
WRT to scaling, this blog post of mine might be generally helpful: Redirect
And while I am at shameless self promotion, check out this app list (and please, if you can, do contribute):
https://linmobapps.frama.io.
Super looking forward to receiving my Librem 5, but given I backed it on October 23rd 2017 I guess I am going to have to wait a while.