This package provides ROS image transport "qoi" which uses QOI image format.
This format is a lossless codec for RGB or RGBA images with similar compression ratio as lossless PNG, but orders of magnitude faster.
This image transport also provides the image_transport_codecs interface. This means it is possible to directly encode/decode images from C++/Python code without the need for going through a pub/sub cycle. You don't even need a ROS master for the conversion.
A benchmark compared to PNG is available here.