An addition to Krista's advice --
After you browse to the pictures you want to add, the last important step after you add the files is to click the "Upload" button in the lower right part of the small "Manage Attachments" window
to actually transfer the pictures to SMF.
This step is very easy to skip since this button is often hidden -- the window doesn't open up big enough to show the button unless I enlarge the window size.
Originally Posted by KristaY
When you "post reply", scroll down to Additional Options. Under that heading you'll find Attach Files. Click on Manage Attachments and it'll bring up a screen where you'll add the files you want. To make sure you have the right photos, click on "Preview Post" before you Submit Reply. That should do it for you!