Here's a very recent
thread (March 2018) about how to best downsize and post pictures on IIS. Sadly it's been archived so the posted images have vanished. But from the remaining words 2 strategies seem to be the way to go.
Either a) export a JPG from your procecessing software size_of less than 2MegaByte. When you upload that into IIS, the forum software resizes it on its own to the limit of 200KB and that result is apparently more pleasing.
OR
b) download and install Irfanview to resize images before posting.