Run a search on the forum for “Removing HTML Margins” and you should find what you are looking for as this has been answered many a time before.
As a hint… you will have to add an image into a .html page and load that .html page into your pop up window, that html file will then use the no-html-margins code in it to remove that border (when you load in an image by itself, the browser defaults to the default margin settings that is why you have to put it in a new html page with set html margin sizes.