How to select a radio button by default?

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>

Use the checked attribute.

<input type="radio" name="imgsel"  value="" checked /> 

or

<input type="radio" name="imgsel"  value="" checked="checked" />