I used to do this sort of thing a lot. It depends whether you want it to look like she's actually been gunged - or somebody has crudely grafted/drawn some gunge onto the picture using proprietary image software (?). In this case, gunge would have to be painstakingly airbrushed in. There are a couple of problems with the original photo, too: 1, it's not very high resolution to start with; 2, it's flooded out face on with light from a flashgun, flattening the whole thing out; 3, although her hands look right, her facial expression is not actually showing much of a reaction! These conditions would have to be matched by any reference material used.
It's actually incredibly time consuming to do well and I stopped for various reasons (certainly celebrities anyway!), but mainly because it simply wasn't worth the effort (8-30 hours for each image!). I didn't go about it quite as suggested above either. In my examples, the subject is amalgamated from multiple screen shots merged with about 20 or 30 fragments from other sources to build up the finished image from layers (re-rendered at 300dpi) It's then 'airbrushed' over and a filter applied to even out the disparities (and pixel aberrations) among all the different source images.
Love you, too
