[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

[FrequentLurker]

Here is another one: https://f-droid.org/en/packages/ru.tech.imageresizershrinker...

It also has a bunch of other features

[tentacleuno]

Obligatory mention: ImagePipe[0]. It lets you compress pictures and edit them. You can share images to ImagePipe and it automatically shows a dialog to share compressed versions with another app (hence the "Pipe" -- it's a pipeline!)

[0]: https://f-droid.org/en/packages/de.kaffeemitkoffein.imagepip...