I'll wipe the tile with rubbing alcohol but that is the only prep for the tile. Then I spray paint it and let it cure overnight.
I open the pics into Corel Paint Shop Pro. I resize the pic and set the Pixels Per Inch to 254.
I then process the pic with a custom script written by Sean Murray who runs "That Dude With A Laser" facebook group. He has one labeled Basic Processes, and one labeled Da Wood Script. The Basic processes script is the one I use for tile.
Join the group and you can find it in the "Units" section.
It adjusts some settings and then dithers the picture. Save as BMP to the desktop.
Load into T2Laser, verify that the size what I want. Select BLACK AND WHITE mode. The pic is already optimized for lasers, to run in dithered mode or grayscale will reprocess the pic.
If I'm engraving on white tile painted a dark color I invert the pic. If I'm engraving on a dark tile painted a light color I do not.
I run about 1500 speed, but different tile and different spray paint will require different power levels. Start low and work your way higher. If you have gaps, change the Resolution from 0.1 to 0.09.