ok, I just copied class "pannelinfo", and add one more line "padding-top: 2px", and rename it to "whosonlinepanneinfo". Then in "class.whosonlinemodule.php", replace "pannelinfo" with "whosonlinepannelinfo". Thank you all.
@litouch thx for the feedback. the simpler way: you can modify your custom.css for the whosonline photos any way you want by adding it to the custom.css of your theme. here's an example based on 422's margin suggestion: #WhosOnline .ProfilePhotoSmall { height: 24px; width: 24px; margin-top:5px;}