"Paint" is very simple software that lacks many capabilities found in more sophisticated image processing software packages, such as Photoshop. However, if you only need to reduce image file-size there is an easy-to-use alternative on the web called Smush.it.
Do the following to use Smush.it:
1) Load your original full-size image into "Paint" and crop its height or width as necessary to make the image square.
2) Resize the resulting square image so that it is 145 x 145 pixels.
3) Save that image to your computer hard drive.
4) Go to http://www.smushit.com/ysmush.it/.
5) Click the UPLOADER tab.
6) Click the "Select Files and Smush" button.
7) Navigate to and upload the 145 x 145 pixel image.
8) Click the "Download Smushed Images" button and save the file to your hard drive.
9) Unzip the downloaded file to recover the "smushed" image file.
In most cases the resulting image file will be significantly smaller than the image file you uploaded without any loss of image quality. However, some original images contain too many different colors to allow their sizes to be reduced below 15KB with Smush.it. Send me a PM if you have that problem and I will reduce the image size for you using Photoshop.
Incidentally, Smush.it is a perfectly safe service of Yahoo! It will not do anything harmful to your computer.
-Bob