[accessibility] add alt-tag for image

app/views/roles/index.html.erb

@@ -16,7 +16,7 @@
   <tr class="<%= cycle("odd", "even") %>">
   <td><%= content_tag(role.builtin? ? 'em' : 'span', link_to(role.name, :action => 'edit', :id => role)) %></td>
 	<td align="center" style="width:15%;">
-		<%= image_tag('true.png') if role.is_a?(GlobalRole) %>
+          <%= image_tag('true.png', :alt => l(:general_text_Yes)) if role.is_a?(GlobalRole) %>
 	</td>
 	<td align="center" style="width:15%;">
   <% unless role.builtin? %>