Xterm should support sixels out of the box but needs further configuration to make it work properly:
XTerm*decTerminalID: 340
XTerm*numColorRegisters: 256
If you want to scroll the content in alternate screens like less instead of the scrollback buffer with your scroll-wheel you have to enable: XTerm*alternateScroll: true
If you want a subtle and less jarring visual terminal bell you can enable this option which flashes only the current line and not the whole screen: XTerm*visualBell: true
XTerm*visualBellLine: true
XTerm*visualBellDelay: 20