You can get various devices to mount the frames in the hardware section of Walmart (I know because I happened to have the same problem recently). Some are wires that have a place on each end to nail (small nails) to the frame. Others are metal with teeth that are also nailed to the back of the frame. 3M makes adhesive tape that you might also use, depending on what surface you want to attach it to.

So I have 2 different types of picture frames. One is a normal picture frame that oddly doesn't have hooks on the back to hang on the wall. and the other is a thing I got from crafts direct. its flat on one side and the other side has little compartments. ill add a picture for better ideas (it is not as deep as the picture). anyways I didnt get to do what I wanted with the compartment frame so I flipped it over and used the back flat side. now im not sure what I can put on there so I can hang it on my wall with a tac or nail. same with the regular frame. any ideas? DIY ideas?