You take both your square pieces and fold them in half.

And then you unfold them and fold in the sides into the centre.

You fold them the same way again the other way and then cut them in half.
Now you have four rectangular pieces divided in four by folds.

(this image sucks, sorry)
You then place two pieces vertically side by side. Put glue on the four outside corners (so not where the two pieces touch each other in the middle).
take the two other pieces and place them horizontally on top of the vertical strips and glue their corners together.

Glue on or draw or whatever your images and you're done. To change pictures you open out from the middle.
I hope this made sense. If it didn't, or if you have any questions then feel free to message me.