You can post images from both PCs and Macs. Not much difference between the two.
NOTE: Click on REPLY.
If you simply begin typing in the empty post space at the bottom it defaults to QUICK REPLY. This can make adding images tricky.
So instead,  click on REPLY.
Then you'll see "Click or drag files here to attach them" at the bottom. (You won't see this option if you're in QUICK REPLY mode.)
.jpgs and .gifs only.