There are no buttons to perform action. Whatever is set in the Edit and Watermark settings will be applied to any file you drag into Bzzt! Image Editor, which will make the changes and spit out the file however you’ve requested in the Output settings.
For example, to convert a file from BMP to JPG, make sure JPG is selected in Output Format and then just open a Bitmap. It will be converted into a JPG and saved wherever you’ve defined in Output Location.
Hope this helps.