Make a large pic (1024x768). Then fill it with a light grey colour. In the coulour palette choose a white background colour. Then go -> Filter -> Stylize -> Tiles... 'Set Number of tiles' to 99, and 'Maximum offset' to 1. Choose 'Fill empty area with' 'background colour' (the white you selected earlier) then press ok.
Now you have the pattern. Open some pic, and drag the pattern layer on to it. Then make it transparent.
If you want bigger tiles, just set the 'Set number of tiles' smaller.