Have an image fill your text instead of a color, appearing inside the letters, with GIMP, Photoshop or any painter supporting layers
To have parts of an image as background of your text, filling the letters instead of a color, is a great effect, and it is an effect you can create easily with a free photo editor such as GIMP.
As you will see, what you can do with GIMP can be achieved with Photoshop or any editor that supports layers.
1) Select the image that you want to appear inside the letters, and open it.
2) In the Gimp toolbar press the Text button and create a text layer, change the text color to white and select a font family. A bolder font type lets a greater part of the image appear inside your text.
3) Type your text.
4) Right click the text layer and select the option Alpha to selection. Right click again the text layer and delete it by selecting the option Delete layer.
In this stage you must have only your background image and your text floating transparent on top of the image.
5) Press Ctrl+C to copy your text, and immediately Shift+Control+V to paste it as a new image.
Now you have your text filled with your image instead of a color! Export it as a GIF, PNG or any type that supports transparency, if you want to keep transparency between letters, or as JPG, BMP or any image type if you just want some color between letters.