OK,Open the picture with the pig
Go to the layer tab and doubleclick in the background layer (with the pig) to create a normal layer from the background layer.
Select the pig (or select the area around the pig and inverse the selection)
press Ctrl J to place the pig on its own layer
Place the file with the picture you want to use to cover the pig on a layer above the pig
Press Alt button while you move the mouse in the layertab between the layer with the pig and the layer with the cover
When the mouse-arrow changes to two circles, click with the mouse.
You just created the clipping mask.
Go back to the layertab. On top you can select the layermode, which is default set to NORMAL. Change it to multiply and see what happens. Depending on your files, one of the other MODES might work better, you just have to try
good luck!

Theo