It's this yellow box above ^. Click on it and then you have to insert a url off your computer pics. I haven't done it in so long I don't know what to tell you. If it says browse I can post from my computer otherwise I copy and paste from photobucket.
If you have a photobucket account or similar online photo uploading website you can copy and paste them were ever you need them to be. Otherwise there are symbols at the bottom of the reply box you'll have to read up and experiment with. That's why I have the photo bucket account so I don't have to go through the whole process on the forums. The account is free and I have thousands of pictures on it. If it wasn't for that I would have lost almost all my photos when my computer crashed and lost everything on it.