Interface And Graphics :: Lots Of Data To Handle - Locate A Tile In The Map When It Is Clicked?

My tile-based map editor project. So far, it works really awesome. Here, look at a picture: [URL] And the screenshot is outdated: Now I got the Pencil, Rectangle and FloodFill tools too!, the tilesets are from that popular (well, not so popular) game engine called RPGMaker.

Currently, this is how the map is structured:
One ArrayList represents the X coordinates.
One ArrayList in each X represents the Y coordinates.
One ArrayList in each X,Y represents the tile.
The tile is an ArrayList containing three elements:
The Image name (Tileset01 in the screenshot), the X coordinate in the image, and the Y coordinate in the image. Those coordinates are from the TILESET01 image, not map.

Getting the idea?
How do you locate a tile in the map when it is clicked?
First I get the cursor's position on the map picturebox, then convert it to a multiple of 32 (the tiles are 32x32, yes), divide the position X and Y by 32. The results are the indexes I need to get to the tile I clicked.

That method works just fine for me.

When the map picturebox invalidates and needs painting... When that happens, I iterate through each X and Y arrayList. When I get the arrayList that represents the tile, I use it to draw on the map.

For example:
Iterating X arrayList.... step 8......
Iterating Y arrayList..... step 3.......
Found arrayList with three elemetns: ("Tileset01",32,96)
Draw image on the map picturebox, source is "Tileset01" on rectangle (32,96,32,32)... draw it on map picturebox's point (8 * 32) and (3 * 32)

And so on.

Now you understand how I am working with my map editor, how I am locating clicked tiles, and how I am drawing it. Everything goes fine, except for one problem: At some point, the user might want to make a 500x500 map.

That means, an arrayList with 500 elements representing the X tiles. Then an arrayList with 500 elements for each X tile, representing the Y tiles. Then an arrayList in each X,Y pair.

Then multiply that result by 5, because there will be 5 drawing layers. How am I supposed to handle such huge maps?! My app explodes when I set the map dimensions to 500x500! 500x500 is the max setting allowed in my app. I also know that 500x500 map editors are possible, because I have seen others before. With great performance.