I've done tile over tile before. here's how I did the prep work undercut baseboards, clean existing tile, remove wax or other coating that are on existing tile, since your tile is not shiny but even if it was scuff it up (you don't have to grind the face off it) any tiles that are high chip just the high part and then fill with a quick set concrete to level that area any low tiles 1/8" or less should be ok once you put the adhesive down to set new tiles anything deeper you should fill in as far as adhesives go use a modified thin set mortar i use Tec TC393 superflex white while the drytime will be extended going tile over tile you will still get a solid bond and then just grout as usual
Zeeya