I noticed that many of the images you used in your matching tiles are from icon collections and the ones I identified were free for non-commercial use. So if you use them you could not sell your game. I have started making new icon renders of various stuff I made myself, this way you'll be 100% sure the graphics are free for both commercial and non-commercial use.
This also meant it will take a bit more time to complete the graphics upgrade, but things are moving along nicely, I got about 30 new icons made so far.
Could you do me a favor and contact me via email (there should be an email button at the bottom of my posts) that way I can toss you the graphics as I complete each step. and if you have ideas to make the game more interesting, just let me know.
Cheers, so far this is fun stuff.