Normally the easiest way is to just upload your imagery to Cesium ion (https://cesium.com/ion) which will handle reprojecting and tiling. In this case it looks like the Mars spatial reference isn’t supported at the moment, I get:
Error: Invalid spatial reference system
File: Mars.tif
Spatial Reference System: PROJCS["SimpleCylindrical Mars",GEOGCS["GCS_Mars",DATUM["D_Mars",SPHEROID["Mars",3396190,0]],PRIMEM["Reference_Meridian",0],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]]],PROJECTION["Equirectangular"],PARAMETER["standard_parallel_1",0],PARAMETER["central_meridian",180],PARAMETER["false_easting",0],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["Easting",EAST],AXIS["Northing",NORTH]]
So you may need to reproject it to something else.
What happens when you pass --profile mercator ?
An example that may be helpful to look at is this Sandcastle showing lunar terrain and imagery. This is from this forum discussion: Clamping To Terrain on Lunar globe. It looks like the WMS imagery of the lunar surface in this example is served as EPSG:4326.