1. Open up your image in Photoshop or a similar program. This tutorial is based on Photoshop, but will work fine in other programs.
I will be using this one from NeonScent's Blends section. You can use the same one:
2. Open up your texture that you wish to use on your image. I will be using this one from NeonScent's Textures section (click to enlarge):
3. Drag your texture onto your image canvas. (Click and hold mouse on the texture, put onto image and let go.) Rename it Texture if you have a lot of other layers.

Position it the way you want. Resize it if necassary.
4. Go to the Layers palette and make sure the Texture layout is on top of the image layout. If it is not, then drag it to the top.
Select the Texture layer and go to Layer Style. Experiment with the layer styles. The most commonly used and effective are Overlay, Soft Light, Screen, Color, and Color Burn. Adjust the Opacity if you need to. Don't worry if it covers the face or anything.
5. Is the Texture too dark in some places or covers a main object? Use the Eraser tool to erase any unwanted darkness. Adjust the Opacity of the eraser tool if you need to.
6. You're done! This is my final product:

The Layer Style is Hard Light, Layer Opacity is 90%.