You must have Windows Explorer set to not hide file extensions for known file types. Find out how to do so here.
There is a less well know feature of LCPDFR that allows you to choose your own background for the police computer.
To use your own background, take the image you want to use, make a copy of it and rename it to PoliceBG. So, if it was called MyImage.jpg or MyImage.png before, it will now be called PoliceBG.
Then place this 'PoliceBG' file into your LCPDFR folder. This is located inside the 'scripts' folder of your GTA installation.
You're done! This new background should show when you next use the police computer system.