Using the Selection tool on the polygon setting make a selection all around the body of the person.
Click Ctrl-C to copy your selection. And then click Ctrl-D.
Using the text tool, now type whatever you want but make it big enough to fill the whole picture. For my font I just happened to use size 68 but it varies depending on your font.
Move the font where you want in the picture.
Now click Ctrl-V and move the body on top on the old body to make it look like the text is behind the body.