Sixel is a terrible encoding. Most of the newer terminals have escape sequences for sending base64 encoded png/gif/jpg--for higher quality output & lower bandwidth.
This is not based on technical facts, but bias against an old format.
> png/gif/jpg--for higher quality output & lower bandwidth
For bandwidth, my ssh stream is compressed.
For quality, I can't see difference in a 24 bit bitmap when comparing with a png or jpg, unless I compress them too much, in which case the compression artifacts start playing AGAINST the formats you recommend.