Photographing and Texturing techniques...
I've done a little work with making textures, but I often use satellite images of specific textures I want. If you're trying to take photographs for textures, its most important to minimize any visible perspectiving of the image (think of how roads narrow the farther away they get in an image.) By reducing the perspective in the image, it is more usable.
As for sand:
And if you wanted to make a 'tile-able' texture, you can use photoshop (and I'm sure GIMP too) and follow this tutorial i wrote attached below.
Also, you can even skip that process and use the Healing Brush Tool (in photoshop) alone by first tiling your image over the space you need and healing the seams as you see fit.. (see attached, again, as i used your image). I hope this was clear? And good luck with your work.