So, if you have a dark background, set it to black, if you have a light background then set it to white. Set this to the colour that MOST RESEMBLES your background, not that which you'd like your text to come up with. On the dialog that then shows, choose between Stretch/Tile/Centre and then on the top right there's a selector marked 'Colour' Basically, open up the image you want to use in firefox (I find all of my desktop backgrounds on the internet anyway), then right click on it and select 'Set as desktop background'. The easiest way I know to do this is by using firefox (not tried it with other browsers). The thing is to change the 'colour' of your desktop background (the colour it would show if you're not showing a picture on it) There is actually a trick whereby you can do this.
