If you want to add photos you've downloaded, go to the "pictures" tab in My Stuff on the side or wherever you android users have it. Then upload a new picture. You may need to size it down.
Then go to where you want to insert the picture, and click the colored polaroid looking button near the bottom of the editor options, and it should pop up there.
If you have a link to the picture, go to where you want to put the picture, and click the black and white landscape button and insert the link where it says URL. You also may want to size it to fit the space.
That is good to know. I had previoulsy used the site to hold all my profile page images and such but I may want to upload them somewhere safer.