An icon for an issue type which uses and absolute path URL will not be displayed in the Custom Field screen under these circumstances:

• the icon is served from an external source and the absolute path is provided, i.e. the URL starts with "http://myhost..."

AND

• the JIRA instance contains a context path (e.g. /jira)

This happens because the image URL is appended to the context path. Relevant part of html from the View Custom Fields page:

<tr>
        <td>Issue type(s):</td>
        <td>                  
                <a style="padding-right: 0.3em;" href="EditIssueType%21default.jspa?returnUrl=ViewCustomFields.jspa&amp;id=8">
                    <img src="/jirahttp://www.google.com.au/intl/en_au/images/logo.gif" title="test " alt="test"></a>            
        </td>
    </tr>