Applet That Allows A Web Client To Do A Screen Capture

I am attempting to develop a completely web-based set of tools that can
match the functionality of a set of Tcl/Tk programs that are used in
our control system environment. I believe that I can do everything that
the Tcl/Tk tools are doing except this one thing:

The Tcl/Tk app allows the user to pick two points on their screen
(which defines a rectangle) and then automatically grabs that potion of
the screen as an image and embeds it into the Tcl/Tk app as an object
which is then uploaded to a database when the user has submitted the
Tcl/Tk form.

I would like to have a web-page with a form that allows file-uploads
(this I can do), but I would like the page to include an embedded Java
applet (presumably) that allows the web-client to capture a portion of
the screen and automatically attach it to the html form which can then
be uploaded to a database when the client submits the form.

An important requirement of this project is that it not require the
user to install anything prior to visiting the web-page. The applet (or
whatever) can be installed but not a separate program. I really want
this entire function to be provided from the web-page. I can provide
more information if necessary, but I though that a broad statement of
the problem would allow more people to chime-in with their thoughts. I
am eager to find out how this might be possible.