An item prefixed with a colon is a symbol. It’s sort of like a string. “class” => “test”, :class => “test”, etc. Without a colon prefixed, it would be a variable.
<%= link_to image_tag("
image.jpg"), :controller => ‘main’, :action => ‘index’ %>
<%= link_to “Text”, :controller => ‘main’, :action => ‘index’ %>
Just replace the text that you want to link to with your image_tag.
You might want to read a few basic texts on Ruby before starting out. Why’s guide is a good intro (and fun), or the first edition of the Pickaxe is also available online, and it provides a good enough “textbookish” coverage.