You can do this by downloading VMKpal at http://www.vmkpal.com
After you download it open it up
Now, press login to VMK click the VMK button at the bottom
Type in your username and password press login
Find a time when you want to take the screen shot and just press the camera at the top
Now, click the photoalbum at the top and click either save photo or crop to file if you want to make it smaller
After you save it just go to a website like http://www.photobucket.com and make an account
Click browse and find the photo on your computer.
Now click upload
Now copy the URL link it gives you and go to the micechat avatar select and paste the URL in the place you are suppose to place it.
Yes, and if you don't want to take time to download VMK Pal, you just press the Print Screen button on your keyboard, and then paste it into a editing program such as paint. From there, you can crop it, edit it, whatever else you want to do, and then save it.
Then after that, go to your User CP, click edit avatar, click browse under upload from computer, and select the picture you want to use.