TheCookie1 sent me a sample picture, and the results are a bit weird.
I got a picture of tomatoes. The picture compressed by Discourse appears “redder” (more saturated) than the original.
If I upload the image to Discord, the image on Discord also appears redder than the original.
But here’s the weird thing:
If I download both the Discourse and Discord pictures (that appear both redder in the browser) on my hard drive and open them both with Irfanview, only the picture downloaded from Discourse appears redder. The picture downloaded from Discord looks like the original BUT appears redder in the browser. 
In conclusion… I have absolutely no idea about what’s going on 
(the original photo isn’t HDR by the way.
Original:
Recompressed by Discourse:
Uploaded on Discord: (I don’t know what Discord does with the uploads, but the final image is slightly lighter than the original)
