shell picture format converter?

[gregor]

i'm searching for something like "Acdsee" has in its own menus. that is the function where you can convert images from one format to another.. you only click the file(s) press right mouse button and select the format.. there it is.. the result.

something like i'm searching for did the MS in powertoys, where you can change the IMAGE size.. that is, directly from shell..

anyone knows something like this, but for image formats??

thank you very much..

gregor