Since that's the format OFP uses, you sadly have little choice in the matter.
1. If you don't have a suitable photo-editing program, try
The GIMP, which is free and highly capable.
2. For your own purposes and in order to learn, have a look in the
Generic Middle East Resistance. There are faces in jpeg format with an approximately correct skin tone to use as a base.
3. Ignore the Back, Side and Hands for the time being and just try to replace the Face section for the time being. Place whatever picture you want to use over it as a 'new layer', but adjust the 'opacity' so you can see the original underneath.
Stretch and move your new face until the eyes, nose and mouth line up with the old one. Once done, return the opacity to 100% and export the picture.
4. Realise that your first efforts are unlikely to be very good ... just like my Che Guevara for online play: