How to zoom picture in asp.net web forms imagebutton

I have an asp.net web forms application which displays a picture of the user's choosing in an ImageButton web control. I am choosing this particular control because I want to record the position of clicks in the display. In my line of work, we use very large images, up to 7200 x 4800 pixels. I have a few questions about displaying such a large image. I want to keep the ImageButton control a decent size (i.e. 900 x 600 pixels), but I want to keep the picture its native size. How do I program an ImageButton to do this? I preferably want to do this in c# or in css.

I already tried this in css:

.main_container {
    blah
    blah
 }
.main_container > .imgbutton_container {
                      float: left;
                  }

I want to show the picture like in the screenshot attached to the question

In this case I just nested the Image control inside the Panel control without setting Image size in the .aspx file.

Also, how can I zoom the image when the user performs an event with the mouse? (i.e clicking a radiobuttonlist item)