I don't know if a post on this has been done already, but lately I've been asked this a lot so I thought, "Why not make a post on it?".
Step 1: Go to imgur.com or any picture sharing website.
Step 2: Click this:
Step 3: Once you upload the image you should have this:
Right click the image and select " Open Image In New Tab". You should get this:
The link is different for everyone. By the way I meant to point the link above the one I'm pointing.
Step 4: Go to wherever you want to post your picture inthe pixelmon forums and select this:
Step 5: Paste the link you copied here:
Step 6: Just click ok and it's done :).
Have a good day,