A "Toasty" is an easter egg where if you get a 250 Marvelous/Perfect combo at any point in a file, a clip saying "Toasty!" plays and a picture pops out from the right side of your screen for a couple seconds. You can edit the picture and sound clip to anything you want, and request to have yours uploaded in the #repo-request channel of our discord
To add a Toasty, navigate to
\Etterna\Assets\Toasties and overwrite the toasty.png, toasty.ogg, and/or default.lua file to the new one you have downloaded.