Okay i am starting off with these two pics of ashley tisdale! Try to find two pics that have a simmilar background (it will look better if the backgrounds are similar).
Please drag images to your address bar so you can see the full image! these images are re-sized so they would fit!

Okay click on the star tool like it shows bellow!

Highlight around the 2ed pic like shown bellow

then drag it onto the 1st pics.....if it has a white background when you move it ontop of the 1st pic go up to image and make sure that there is not a check by "Draw Opaque" if there is then click it and it should go away! bellow is my finished product!! Hope this helped! if you have any qustions or need any help on this please post in the in the c-box! thanks!