I was hoping PNGGauntlet 3.1.0 would be the last release I'd need to do for a while, but it looks like there were still a few bugs that needed to be fixed. 3.1.1 is out and fixes all the bugs that were reported to me. The most important was a bug where you could add a non-PNG image to PNGGauntlet, and after compressing the image would convert to PNG, but still have its old file extension. Now, the file extension will be changed to .png as you'd expect. Besides that, I've made it so that PNGGauntlet will run on the smaller .NET 4.0 Client profile, and I've fixed the “Cancel Optimize” button so it'll cancel immediately, killing any in-progress compressors, rather than waiting for the current compressor to finish. That's particularly helpful since some images can take minutes to compress. Please grab 3.1.1 if you haven't already, and feel free to email me if you notice anything still broken.