How do you replace a LOD file?

So I wanted to replace a tracks standard load screen with a black and white photo image. The original GTL .jpg file was 800x600 pixels so, using a image editor, I made the replacement .jpg the same size. I renamed the original file and gave the replacement file the same name as the original. As far as I'm concerned this should have worked but it hasn't. What have I missed?

Ok, looks like I found what was stopping it from working. I had to change the Vert and Horiz resolution to 96 dpi. Works now.