Add image to OpenSearch description

[squiddy]

We should include an image, it is displayed when selecting a search provider in Firefox for example.

Probably make it a setting for admins to point to a custom image, otherwise fall back to a richard-specific.

When possible, search engines should offer a 16x16 image of type "image/x-icon" or "image/vnd.microsoft.icon" (the Microsoft ICON format) and a 64x64 image of type "image/jpeg" or "image/png".

http://www.opensearch.org/Specifications/OpenSearch/1.1#The_.22Image.22_element