Custom search engines cannot display icons, and icon uploads are not available.

[shtianfa]

Custom search engines can

not display icons, and icon uploads are not available.

[Jon]

Thanks for submitting an idea to the Mozilla Connect community! Your idea is now open to votes (aka kudos) and comments.

[Standard8]

The icon will automatically be set to the favicon of the domain when the search engine is added - make sure that you've visited the site first to add it.

If you've already added the site, you can visit the page and then edit the search engine, which should update the icon again.