You should seriously consider doing this for your mobile client; the worst thing about Discord is that it eats mobile data if you're uploading lots of images.
Data aside, my phone takes pictures at ridiculously high resolutions. Whenever I go to send pictures with discord, it takes a good 30 seconds and half the time it'll just break.
As an aside, I wish the "share" button would share a lower resolution image instead. I don't mind storing the full quality picture, but handling a 10mb image is seriously silly.
The other side of this is if you're on mobile data and they decide to resize the images on the client then people will complain about the app eating up battery life because it's using so much CPU to resize the images. Plus, they won't have the full size image to share with people you may be chatting with on desktop. If you want to not use so much data while uploading images then you should probably resize them yourself or just not upload images unless you're on wifi.
