There are several ways to do this. Everyone should post up their preferred method, and I will clean it up and make a sticky since it is asked at least once or twice a month.
The way I do it:
1) Download the free program for image manipuation at
http://www.gimp.org/ You may need to download other programs to help it run, but the tutorial is straight forward.
2) Launch GIMP, click on file -----> acquire -----> screenshot. Select Grab Single Window.
3) Move crosshairs to the window you want to copy. GIMP will open a window where you can manipulate the image, block out anything you want to block out such as account name,etc.
4) File ----> Save As. Select the folder where you want to save the image. Name the file, using a proper extension. The naming convention I use is YY.MM.DD.text.jpg so my folder is in chronological order, and there is some descriptive text so i know what is in it. You can come up with your own naming convention, but this works for me. Be sure to end with .jpg file extension (or some other permissible type of extension for the web service you use).
5) Open
www.photobucket.com. Make an account if you don't have one already. Go to Upload images. Follow the instructions to upload an image from your computer. Select the image you want uploaded. Photobucket will add it to your album.
6) Start a thread at BTP. To add your image, simply type "[img]URL%20of%20your%20image%20at%20photobucket[/img]" minus the quotes.