after running our website for a year we ended up with about 200'000 pics on our servers, because each uploaded image had about 9 different formats even tough we didn't need them all for the same image. normally you need 3 sizes per image: thumb for backend, actual size for frontend, original size
this plugin solves the problem perfectly, you can even delete all the old pics and they will be regenerated by the plugin when needed.
the specific image formats are already being generated when opening the preview, so in most cases "on the fly" doesn't mean it will be triggered by one of your readers but by yourself when editing.
before using the plugin go through the support posts and apply the fixes that are suggested by the other users.
and get rid of the "c" in the filename!