Convert BMP to PNG on right-click in Windows 7

Getting something with one click is always tough, but this might be what you're looking for.

XNView

Use it in conjunction with the shell extension, which offers a host of image manipulation options on right click.

Addictive Tips: XNView shell extension

Previously called "Right Click Image Converter", ImageBadger is an image converter which has the capability to convert multiple images with right-click.

The program is a shareware. There's a try & buy version on the official site.