When you click on 'Post Reply' and the box comes up, scroll down and look to the left in the green part.
You will see a box labeled "Manage Attachments". Click on that and it opens a window with 8 or 10 boxes labeled "Choose File".
Click on the first one and it takes you to your computers Document Library. Find your pictures and double click on the one you want. If you want to upload more than one, click on the next box and repeat.
When you have all the pictures attached, click "upload" in the right bottom corner of the manage attachments window. You should have pictures attached and you can then click submit.