As other have mentioned you can't display an image in an alert. The solution is to show it on the webpage.
If I have my webpage paused in the debugger and I already have an image loaded, I can display it. There is no need to use jQuery; with this native 14 lines of Javascript it will work from code or the debugger command line:
function show(img){
var _=document.getElementById('_');
if(!_){_=document.createElement('canvas');document.body.appendChild(_);}
_.id='_';
_.style.top=0;
_.style.left=0;
_.width=img.width;
_.height=img.height;
_.style.zIndex=9999;
_.style.position='absolute';
_.getContext('2d').drawImage(img,0,0);
}
Usage:
show( myimage );