Checkbox when grouped

You're missing the brackets ( { ).
if ($value=="yes")

{

$value = "<img src=\"images/check_yes.gif\">";

}

else

{

$value = "<img src=\"images/check_no.gif\">";

}

No that didn't work either.
I copied and pasted your code into the custom code and the output is still <img src="images/check_yes.gif"> instead of the image itself

It looks like your code should have worked the first time. I can't see what the problem is. Maybe someone else can help.
Display image

Try to upload your project to the demo server and check if you can reproduce the same issue.

I assume that you are using this custom code for the list page. If yes, could you try this one?

Replace your_field_name with actual field name.
Good luck!