A lot of artists on GASR require IMVU screenshots in their order forms, this is a guide on how to create the 'perfect' screenshot
For this guide you'll need the IMVU client which can be downloaded here, you'll also need an IMVU account to access this.
In the IMVU client, open up either 'Shop' or 'My Room'
You'll need to take the screenshot in a shadowless room, if you're in the Shop you can just add one in. If you're taking screenies in your room then you'll need to purchase a room, this can be found by searching 'shadowless' on the rooms tab in Shop.
Ensure that your avatar has all the items that you want them to be wearing for your screenshot. Many artists prefer the screenie to have minimal accessories, which they can draw in afterwards but ask the artist first. You'll also need to consider whether the artist accepts bald or screenies with hair. If bald - you can search in the shop for a 'bald' head under hairstyles.
Unless you're ordering a full body edit - it's usually better to take the screenie from around waist-up, so zoom in by holding down your mouse and scrolling up or pressing the plus key on the top right hand side.
Type into the IMVU client '*hiresnobg' and press enter when you're ready. This will save a screenshot of your avatar onto your desktop which can then be uploaded onto an image hosting website. Many tend to use DeviantArt's sta.sh.
I also have a video version of this tutorial if that is easier to understand!