Normally, Mobile phone support only small resolution Wallpaper. The resolution of Desktop PC or laptop PC wallpaper are having large resolution (in range of 1280X1024 pixel ). So you can not just copy the wallpaper image file of your PC into your mobile and make it Wallpaper of your mobile (if your mobile does not support high resolution image.)
Here is the easy way to covert large resolution PC Wallpaper to small resolution (240X400 pixel, 240X320 pixel etc ) wallpaper for compatibility for your mobile.
If you are using Windows XP, Vista, or Windows 7 operating system, use the following techniques.
Step 1- Copy the desired PC wallpaper image to desktop.
Step 2- Open “Paint” from Start > All Programs > Accessories.
Step 3 – Reduce the white working space to smallest (5mmX5mm)
Step 4- In the paint go to Edit> Paste from, select the image.
Step 5- When image will come on “Paint” press “CTRL+W” or click Image>Stretch/Skew
Step 6- Reduce the size of Horizontal and Vertical by 65% both (or less if required).
Step 7- Save the image as JPG format.
So the output file is of reduced resolution (240X400 pixel, 240X320 pixel etc )which will be in most cases compatible for your mobile phone to see it or use it as Wall paper.
You can reduce further resolution by above method if required.