Sometimes I will save some screenshots of the website for later viewing. If these pictures have a very large height, it is not easy to scroll. If there is a minimap like Sublime Text has, It should be very good.
Not a bad idea.
At the moment, you might find it worth trying the Fit To Page (or Grow To Page) mode and then setting the mouse button to Expand/Scroll mode. That gives you an overview of the whole image, with quick access to the full-size image just by clicking where you want to view and/or dragging while clicking.
Waiting for your good news for this 
.