Wallpaper /Screen Resolution

Dear Colleagues,
I have saved some *bmp images and *gif images which I use as wallpapers on my workstations. My problem is that when I increase the resolution in Display>settings>Screen Area to anything graeter than 800 by 600, the image I am using as the wall paper starts splitting into 2 images overlapping one another, the greater the resoultion the further the overlap until at 1600 by 1200pixels it completely splits into 2 images. Does anyone know what causes this.