Okay. Before you File -> Save As...
Resizing in Paint
1. Paste your desktop
2. Click Image -> Stretch/Skew
3. Enter in 50% in both boxes where it says 100%
4. File -> Save As...
Resizing in Photoshop
1. Paste your desktop
2. Click Image -> Image size...
3. In the first dropbox, change "Pixels" to "Percent"
4. Type in 50% in the box next to that dropbox.
5. File -> Save As...
Sorry, can't help with MACs.
This is my signature.