We have added a new section, Application Icon, to the /app-key page; the new section includes an input field into which you can save the URL of the icon that you would like to use.
The icon you submit will be used to replace the placeholder icon found on the authorization flow. If no icon is submitted, we’ll continue to show the default icon.
Also, I realize that the name of this isn’t appropriate as we’re not allowing you to “upload” an icon. We’re allowing you to set the path of an already hosted icon that we will use when needed.
If you go to https://trello.com/app-key/ we send the PUT to the wrong URL: /app-key/1/applications. However, if you go to https://trello.com/app-key (notice the missing trailing slash) we correctly PUT to the correct route: /1/applications.
I’ll get a fix out for this shortly. In the meantime, make sure that you are trying to set the icon via https://trello.com/app-key