There is a free (as in [freedom](http://www.gnu.org/philosophy/free-sw.html)) application [Send Reduced](https://f-droid.org/en/packages/mobi.omegacentauri.SendReduced/), available at the [F-Droid](http://f-droid.org/) repository of free software.

Install F-Droid, install Send Reduced, select your photos, press Share button, select Send Reduced and then select the sharing method. Your photos will be sent reduced in resolution and file size.