Dexter is a piece of software that lets one recover data from printed graphs. It can be used online (and with PDFs and such) on Dexter for your Data (or as part of the ADS, if your browser still can do applets).
Here, you can obtain the Java source code, which contains a HOWTO.standalone explaining how to use a "full" Dexter (with PDF and Postscript support) locally.
On the other hand, if you have PNG, GIF, or JPEG images of your figures, you can just download Debuxter.jar. If you have a Java VM installed (try OpenJDK in case of doubt), you can just run this by doubleclicking and perhaps going through a number of nagging questions from your system it should also ask for just about any javascript you execute.
The local version doesn't let you open the help right now, so please have Dexter Help open next to it.
Note that Debuxter, really, was mainly intended for debugging originally (like, 20 years ago). It's never been properly polished with non-technical users in mind. If you're seeing too many wires, feel free to complain.
If your images are large, Dexter may run out of memory. In that case, you might try to give it more memory, most easily from the command line, like this (to give Dexter 900 MB of heap):
java -mx900M -jar Debuxter.jar my-largeish-image.png
If you would like to hack on Dexter (patches are always welcome!), please check out our subversion repository. Yes, this is http for anonymous checkouts.