Hi, I just did this for my family for Christmas. There are 2 ways. A friend of mine used card stock, and regular modge podge. A layer underneath and a layer on top. The lines were very obvious from the brush, so she made designs with the brush, waves, diagonal lines, etc. I think she put a layer on top too. She also tore the edges of the card stock and printed in b+w to give it an antique look.
I used photo paper, and hard coat modge podge. my lines were also obvious, but not as obvious as hers. I also made designs with my brush to make it look intentional. After I adhered my photos to the tiles, I gently sanded the edges of the photo (with a fine grit sanding sponge) for a distressed look. I also printed in b+w. I cut the photos to almost the edge of the tiles before I glued them down. My friend left a little over 1/4 inch border around her photo. Don't forget the little felt pads underneath the tile if you intend on them for coasters. Good Luck.