I see Ghostty does not support (and does not plan on adding support for) Sixels, instead preferring the Kitty image protocol. Now if the Kitty image protocol is so great and the Sixel stuff is so bad, ~~why is it only used in Kitty and Ghostty?~~ *Edit: it's also supported in Konsole, WezTerm, ... but still I'm interested in why we have 2 competing protocols right now.
IMO none of them are particularly useful. Sixels is hilariously inefficient. Kitty is slightly better because you can send data as PNG, but ... you have to send image data as PNG! I wish there was a high performance way of remoting graphics over SSH. How cool would it be if you could SSH to a remote machine and it just showed you the remote desktop in the terminal itself? No messing around with port forwarding, weird…
I think there's at least three different experiences here, and they're all valid, but I don't know what you really want.
A) remote desktop --- connect to a fully formed desktop environment (with SSH to authenticate, I guess?), possibly persisted and/or shared so you can connect back and get into the same place or share with another user?
B) run a program remotely and display it on your local terminal; essentially remote X, but I gather you're looking for more performance and maybe some other nice to haves? Maybe you want to transport audio too... Maybe you don't want the crap experience remote X has become since app developers don't spend any effort on it and you kind of get what you get, which is a lot of jank.
C) images in the terminal, with high performance. PNG should be ok for that, right? Maybe an extension for lossy compression might be nice depending.