Adjusting HTML w/JS from an IMAGE Size

I currently have a popup window to display an image which is supplied via a URL. The URL is supplied by users of my message board.

I would like to have a window popup with dimesions just a wee bit bigger than the image itself.

Basically have JS first find the image get its size in pixels and return that variable to the popup window size attribute.

Is there any functions or routines that can retrieve and write an image size (width = "" hieght = "")to a varaible/string ? I think from there I could taylor my code to achieve my goal.

Thanks for any help !