You first gotta submit your stickfigure using the submit feature in the app.

Once you do that, log on to the site and use the “Submit Thumbnail” page

enter your email (preferably the email you used to submit the figure), Name the figure you want this for, and send it. There are some rules tho, the 2 most important ones is 1. The thumbnail cannot be over 1.0 MB. 2. The thumbnail must accurately be able to show the figure you get. AKA you can’t have a thumbnail with one figure and then get another.

If your thumbnail gets rejected, the figure will still be submitted to the site, but Ralph will make a boring, default thumbnail instead.