How to change facebook app icon

https://developers.facebook.com/apps/[your-app-id]/app-details/ -> "logo" is the way to go. If you uploaded a logo a while ago, it may look like the option to change it is not available, because your old logo isn't showing up (it's just a standard facebook image). That's because your old logo is probably too small. Upload a new logo, and your logo will change.

Info: It's not necessary to submit your app for approval first.

Update: Since "logo" is no longer listed as an option, I assume they have changed the setup form slightly...

New link: https://developers.facebook.com/apps/[your-app_id]/app-details/.

Under the section: "Icons"... Heres the docs

Scroll to the next section to see something which might be of interest too: Promotional images (Cover image & Banners).

NOTE: You may have to do some image editing before uploading to meet the dimension restrictions they have in place (at the time of this post: 1024 x 1024 and size <= 5MB).