You want to start off by going in a MovieClip. Add a new symbol to your library of type "MovieClip", name it whatever you like. Put your 15 frames in there. Now you have a self contained animation.
Go back to your main timeline and you can drag as many instances of that MovieClip you just made in and place them anywhere you like, change each instances properties, etc.
Your problem is you probably did this 15 frame animation on the "Main" timeline and you can only select one frame to "Convert to a MovieClip". You just need to bring in (manually) all the other frames into that new movieclip. Putting your cursor on the timeline and pressing F5 will expand the timeline and give you frames for your additional images. You're going to want to select all these new frames, right click on them and use the convert to keyframes option so each frame will not contain the picture in the previous frame.