To do that, you want to add an attachment.
1. Go to the thread you want to post pics to.
2. Click "Post reply" (just like you did to reply to this thread)
3. Before hitting submit reply, scroll down further to the "Additional Options" box
4. Click on the "Manage attachments" button
5. You'll get a new window popup
6. Click on the choose file buttons, find your files, then click upload
7. Finish your post as normal.