Open the two images as layers. Or open both as images and copy the dirt image to the eye image making sure the dirt image becomes the top layer.
Make sure the dirt layer has an alpha channel. Right click -> add alpha channel.
Rescale the dirt layer to the size of the image with the eye.
Make sure the dirt layer is on top
Set the layer mode of the dirt layer to overlay.
Make a selection around the eye.
Make sure you are on the dirt layer
<-important!!!Then press delete (or go to edit -> clear)
p.s. Try some other layer modes on the dirt layer. You can get some really cool effects.
