First you should check if the size of the mask and the picture you want to use it for are aproximately the same size. - Open the two files - Size the mask to the picture (mask file) - select all and copy - Double-click on the picture (photo file) - Add a mask (circle in square) to the picture - option-click (I think also option-click on PC of alt-click) on the mask [what should happen is that you 'jump in' the mask and your screen turns white] - paste If you click on the picture (or option-click) you should return to the picture, which now has the mask applied. You can resize the mask afterwards, but be sure to click on the little chain picture, to prevent the photo from sizing too. If this does not work out, you can also paste the mask in a new alpha channel and make a selection from that. But that sounds a bit to overwhelming for a newbie, I presume. Good luck!