Easier to explain it than to locate the instructions…
In the toolbar above the message box, click on the paperclip icon. This will take you to a new tab or window called "Manage Attachments." Go to the section that says "Upload File from your Computer." Choose up to four photos, then hit "upload" (way over at the right). Be prepared to wait a while, since larger photos will be automatically resized. Once it's done, return to your post.
Now the confusing part. You won't see the pictures, because they haven't been inserted into your post yet - they are waiting in a queue. Place your cursor in the post where you want the picture to appear. Then hit the paperclip icon again. This time a drop-down menu will appear, which should have all the pictures you uploaded. Choose the one you want. You still won't see the picture, but you should see something pop up into the post indicating an attachment. Repeat for each photo.
NOTE - I think it may be possible to repeat the above two steps if you want more than four photos in a single post, but I am not sure - never needed to.
You're done, but I would recommend using the "Preview Post" first to make sure everything is the way you intended. Now you should actually see the pictures. Last, hit "Submit Reply."
Hope this helps!