Do you mean one you have created in the editor? If so, do this -
1 - Get the players unique ID in-game(Preferences->Display & Sound->Check the box to show unique IDs), and make a note of it.
2 - Get the face you are adding(180x180 pixels max), and rename it with the players Unique ID(12345.png for example)
3 - Then, you need a config Use mine that is attached if you dont know how to make one
<record from="12114" to="graphics/pictures/person/12114/portrait"/>
Open the config with notepad, and you will see some text. Look for the line that looks like the one above. Where the Italic and underlined numbers are, put the file name of the picture in there, without the file extension at the end. Where the bold is, put the players unique ID number there. Save the file, and put in a new folder along with the picture.
4 - Put the folder in Documents->Sports interactive->FM10->Graphics->Faces
You may need to create the Graphics and Faces folder.
5 - In game, go to Preferences->Display & Sound, and check 'Always Load Skin On Confirm', untick 'Use Skin Cache', then hit confirm.
If done right, the face should be there.