With dojo, it is possible to draw arbitrary shapes and then use styles to place the drawings at arbitrary locations on a web page. This means it is possible to use dojo to draw on top of an image in the web browser client (as opposed to manipulating the image before delivery on the server).
Create a drawing surface, then draw a shape (a circle in this case) on that surface.
Create an enclosing container with its position set as relative, then include the image within it, then absolutely position the drawing surface so that it's upper left corner is at the same place as the upper left corner of the image. Drawings can now be created on the drawing surface and will overlay the image, with a coordinate system that starts in the upper left corner of the image and has x increasing to the right (in the image width direction), and y increasing downwards (in the image height direction). As we are drawing a circle, we specify its coordinates with cx and cy for the center of the circle and r for the radius.