To save your image as a different image format call the format method and pass in the desired format. Currently the following formats are supported: FORMAT_JPG, FORMAT_PJPG, FORMAT_PNG, FORMAT_GIF and FORMAT_WEBP.
Alternatively you can change the image format by saving the image with a different file extension than the original image. The Image package will then attempt to convert the image to the correct image format.
->save('converted-example.png'); // Will convert the original image to PNG