Comment by MayeulC

These apps do exactly that, but are not easy to find from the F-Droid app (I had to use a search engine to find the first one):

https://f-droid.org/packages/mobi.omegacentauri.SendReduced

https://f-droid.org/packages/com.caydey.ffshare

Oh, and also, specifically for PNG optimization: https://f-droid.org/packages/com.wrmndfzzy.atomize