This is a rather simple tip, but I have had many people ask for it, so here you go:

	Open up your picture or texture. Select all (CTRL+A) and copy (CTRL+C). Now press your delete key to wipe out the image.
	Go to the channels pallette and create a new channel, now type your text or create your B&W image.
	Click on the RGB button and then go back to the layers pallette. Now load your selection (CTRL+ALT+4) and go to EDIT...PASTE INTO. Now you have your texture inside of your selection.