choose an S ROM file or a C ROM pair from a game's romset, or an SPR or FIX file from a CD game's ISO in the file chooser above
Neo Geo ROMs are usually bundled in a zip file. Unzip a game and you'll find a bunch of files. You are interested in S files or C files.
Or if you are using a CD game, you are after SPR or FIX files inside the ISO. If you have an actual CD, usually loading it into your PC's CD drive is enough to get at the files.
201-s1.s1 in Metal Slug or
019-s1.rom in League Bowling
Note that newer games (from KOF99 onward) don't have an S ROM file. Those games can't be viewed.
019-c2.rom for League Bowling, or
RBFF1_C2.rom for Real Bout Fatal Fury.
Bigger games will have more C ROM file pairs. Real Bout Fatal Fury has C1, C2, C3, C4, C5, C6, C7 and C8 ROMs. You can grab any pair, as long as they go together. C1 and C2 go together, C3 and C4 go together, and so on.
Note that later games encrypted the C ROM data. If the tiles all look like television noise, that's what that is. This viewer cannot decrypt the files.
Bigger games will have more than one .spr file. Just choose whichever one you want to look at.
The data inside C ROMs is for the main sprites of the game: the characters, backgrounds, bullets, etc. The data inside the S ROMs is for the fix layer. This layer is drawn above all sprites and it never scrolls. So S data is for things like the current score, number of lives, etc. The "HUD" if you will.
SPR files are the same as C files, but for CD games. FIX files are the same as S files, but for CD games.
The tiles are stored separate from the color palettes. There's no good way to get the color data from a ROM without running the game. So an alternating blue/yellow palette was chosen as it allows the different colors to stand out.
For more info on tiles and palettes, this blog post I wrote goes into more detail.