Alright, here's what you do.
Create a new movieclip that's empty, and export it for actionacript (right click, Linkage...). Also, the "Linkage" for the MC you want should be "my_mc."
Create a new layer called actions. Create a new keyframe in frame 15 and type the following code into the Actions - Frame thing:
And in frame 25, keyframe:
Something like that. Obviously "mySecond_mc" is the second movieclip to be attached.
And, if you wanted an image, import the image to the library (file > import) and do the same linkage.