There is, but if I understand the comments right it doesn't work in plugin.
Code:
## jpg or png
## -- jpg produces smaller images, and may be supported by more
## readers, but it's older and doesn't allow transparency.
## Transparency removed as if remove_transparency: true
## -- png is newer but does allow transparency, but only in CLI.
## It doesn't work in calibre PI due to limitations of the API.
convert_images_to: jpg