If you've created a custom face, you'll most probably want to show it off to friends and foes. There are two ways to take a self portrait: the easy way and the hard way. The hard way might be worth it.
The easy way is to simply click on your player name on the top left-hand corner of OFP's main menu, select your player name that's using the customized face image and click on EDIT. This will take you to the EDIT PLAYER menu, where a revolving model of your soldier's face can be seen on the screen's left-hand side. Simply take a screenshot here and now. That was easy, wasn't it?
The hard way will require you to enter an existing mission where you command your team or use OFP's mission editor to create such a mini-mission just for taking the portrait. If you choose the latter, create a mission with yourself as one of the soldier's or civilians you would like to appear in for the photo shoot and (you'll see why soon) a jeep or car manned by people on the same side you're on, parked nearby your starting location.
From the mission editor, preview the mission. Hit the DELETE key on your numeric keypad to switch to command view. That should give you a view of yourself from behind. Move your mouse to the extreme left or right to swing the view around to in front of you. Use the numeric keypad's plus ("+") key to zoom in on your face and adjust the angle using your mouse.
Now, you could already take a screenshot at this point and you might even be satisfied. However, when you're standing right next to an object, such as that jeep or car I suggested, OFP will at certain angles increase the zoom so that you can get a better look at yourself and produce a better screenshot. I usually find this works by standing right next to and facing the hood of a jeep or a car. That's how I got the portrait below, for example.