I made the cardstock about 1/8" larger than the pictures. It serves two purposes. One, it frames the picture and makes it look nicer, and two, you'll be able to hide the ribbon between the two layers.
Next, I cut a piece of paper to fit inside the bottom of the tin.
Then the ribbon was taped to the bottom of that piece of paper.
Put extra tape on the paper all around the ribbon to help it stick to the bottom of the tin. Then set the paper in the tin and press firmly all around so the tape sticks.
Allow about 3/4" (this doesn't have to be exact, but you don't want it to be too long) of the ribbon to hang out and lay the cardstock for the first picture under the ribbon.
Cover a good portion of the ribbon as you tape it to the paper. Press it down pretty good, because you don't want it to slip later.
Put several pieces of tape on the back of the picture and place it over the ribbon. Center it on the cardstock so that it's even all the way around.
Barb