Upload a picture you want to use, drag it to the right position and adjust the size using the white circles. To reset the ratio click the button below.
You can either download all five pictures in a .zip file or upload them to Imgur and send the link to the other people in your stack. To upload a picture from a URL you have to click on 'Upload' and enter the URL where the filename would be.
To use CSGOStacks.com you and the people in your CS:GO party just set the generated pictures as avatars on Steam.