Chipsets (aka background tiles)

Each background tile is 16x16 pixels.  These tiles are arranged in a larger file called a chipset, which is a .png file at 480x256 pixels that gets imported into RPG Maker.

These tiles generally fall into three types: your average joe tile, such as grass and dirt; animated tiles, such as water; and objects on a transparent background.  These have to be arranged into a chipset in a certain way so that the program can find what it needs.  (You wouldn't want a random grass tile in the middle of your water animation.)

Here's an example of what a chipset looks like from RPG Maker's freebie files:

Here's a template by a very helpful person (jojogape.deviantart.com), and then I'll explain the color coding.

Blue - Animated tiles.  Each animation gets three frames which run left to right.  The dark blue tiles are for animations with four frames each, running top to bottom.
Green/Grey - Ground textures (flooring, dirt, trees, hills, etc.) that are arranged a certain way in 3x4 blocks.  More on this in a minute.
Yellow Green - Anything you want.  They will lay down exactly as they appear and can be arranged into that area of the chipset any way you want.
Yellow - These are the objects on a transparent background.  Some objects may take up more than one tile.
Black - Leave these completely blank.  A blank tile can be used as an eraser to cover up a tile that isn't wanted.

Tiles in 3x4 blocks:

These tiles are for gound cover that have a background/boarder of a different ground cover, such as the one above which shows sand surrounded by grass.  The program uses all of these tiles together so that, no matter where you lay down that particular ground cover, it can round out the corners and make everything look smooth.  The first tile should show what the ground cover would look like if only a single tile were colored with it (it should have a complete boarder of the second ground cover).  The second should be entirely the secondary ground cover.  The third is entirely the main ground cover, but with the corners rounded off.  The rest of the tiles are laid out to create a 3x3 square with a boarder around the outside.

When a map is created in RPG Maker, it is assigned a certain chipset to use and cannot access tiles from any other chipsets, so each chipset should contain all the tiles needed for that particular area.  Don't worry about what tiles can be walked on and what can't.  That isn't determined by the layout, but is adjusted inside RPG Maker.

I'm going to dump all of RPG Maker's chipsets on here as a reference, since it may be easier to alter some pre-existing tiles than to make them from scratch.

I'm going to explain how to import these into RPG Maker so you can test out your handiwork, but it's a bit of a long explanation (not difficult, just a lot of steps), so that'll be in a different post.