You cannot use a property textthis way - the line textends with "before the word icon.
However, any HTML markup will not be escaped, so you can use the tag <img>.
<asp:Label run="server"
Text="Please see the <img src='icon.gif' /> below" />
From MSDN (Label.Text):
The Text property may contain HTML. If so, the HTML will be passed unchanged to the browser, where it can be interpreted as markup, not as text.
asp:image Label.