Fun With Tiles and Blocks

Well it didn't take too long to get utterly lost again.  Rather than follow the plan and do some game state stuff, I decided block prettification was suddenly priority one. Since my blocks (arranged in a 5x5 square on my .png) all looked the same, I made the happy assumption that my game was just doing what I expected it to be doing behind the scenes, presenting a nicely random array of blocks that all just happened to be identical in appearance. Once I started adding some details, it became apparent that the game was only ever painting a small subset of the available tiles to the screen. With some detective work, I determined that I was getting five tiles in a diagonal row from the top left of the image to the bottom right. Here's an image of the .png, with a number layer overlaying it. The yellow circles indicate which of the tiles are actually being drawn to the screen:

Now if you look closely at the top screenshot, you can see I'm outputting an additional small number at the upper left of each block. This number represents the "tile sheet index", it is randomly bouncing around between one and twenty-five, just like it's supposed to. That's the number that I thought was telling the game which tile to draw, but obviously I was wrong about that.

This is the kind of fiendish mathematical bullshit that serves as a small barrier to entry for the novice game programmer. If you're just grabbing code you don't understand and plugging it in, when it stops working how can you fix it? You don't know how it's doing what it's doing. I really do want to get this fixed but I may have to shelve it in favor of some problems I know I can actually solve quickly.