How to draw a grid over and image that won't be printed? In high school, I used to print images, draw grids over them and carry them around in my sketch book. Now, I have my smartphone with me all the time and am almost never near a cheap and easy printer. Needless to say, I usually use my phone to find art projects or inspiration images.

Is there a software or website that can quickly draw a grid (of my chosen size) over an image on my smartphone that can be saved to my device? It would make my art projects and sketching from a photo a lot easier.
A low tech solution is to print a grid onto acetate, cut it out and just lay it over the image you want to copy. This has the advantage that it will work on any medium and is quick and easy to use once you have made your grid. You could also have different grid spacing for different applications. 

You could also glue it into a card mount to make sure it lays flat.