Question: How can I add an icon to my Rails application (to show in tabs when opened in a browser, etc.)?

Answer: 
You're talking about a `favicon`.

Add this in the `head` part of your layout:

```
<link rel="shortcut icon" href="/path_to_your_pic"/>

```
After reading and trying all this answers without success I ended up doing this

1. Add a file **favicon.png** to your **public/assets** folder
2. In your **$/app/views/layouts/application.html.erb** modify the content of the `<head>` tag adding this `<link rel="icon" type="image/png" href="/assets/favicon.png">`

No need to change the web\_server configuration file (nginx, apache, etc) no need to precompile the assets. 

Just stop and run/debug your web site, clear the cache from your browser and reopen the page. It should work