If you photo is already on a photo host site, open the image to full size. Up in the Address bar is the photos URL. Highlight that, select EDIT>COPY
Go to SMF and start your post. The little Mountian sig above is the one you want. Once the box opens do a EDIT>PASTE, then save. Photo should show up.
You can also type [img] [/img] in the main body of your post and place the cursor between the center two brackets, do a EDIT>PASTE, then save.
Hope that helps.