XHTML solution:
<input type="radio" name="imgsel" value="" checked="checked" />
Please note, that the actual value of checked attribute does not actually matter; it’s just a convention to assign "checked". Most importantly, strings like "true" or "false" don’t have any special meaning.
If you don’t aim for XHTML conformance, you can simplify the code to:
<input type="radio" name="imgsel" value="" checked>