In order to keep the correct face size in a game, face's resolution in the game should also fit the boundaries. That's why, to avoid errors in face preparations, we made a template with correct settings. Download it now. We'll use it later.
Add alpha channel (transparency) to the image layer.
Use free select tool. Select the face, keeping background selection at minimum.
Invert selection. Clear selected area. Deselect all.
Remove the rest of background with eraser tool, with sharp edge, changing brush size to fit your needs.
Open GIMP template (from step 1), copy face photo without background to template and adjust face size to fit between the guides. Use scalling with settings: keep aspect ratio and Interpolation Sinc (Lanczos3).
That's all in this article. But don't turn off the GIMP yet. You need to separate jaw from face and upload both files to PhotoGames server. It was covered in the next article.