There are a number of ways of reducing the file size of a picture including:
- using the crop tool to chop of any unwanted edges;
- using the resize tool to reduce the no of pixels;
- saving it as a compressed format such as .JPG/.JPEG (will reduce quality).
For details on how to use Microsoft Paint please see:
http://windows.microsoft.com/en-GB/windows7/Using-Paint .
It is also possible to further compress a .JPG image than the standard setting in Paint. However, the more you compress a .JPG file the worse the quality gets. You would need 3rd party software to do this anyway.
You could also try using a lossless compression method (doesn't reduce quality) such as zip to make your file size smaller, however there will be a limit on how small you will be able to get your file. Please see:
http://windows.microsoft.com/en-US/windows7/Compress-and-uncompress-files-zip-files .
As with all picture-editing, do not work on the only original file you have. Make at least one copy and work on one of the copies in case you make a mistake.