How can I edit graphics?

You can use Ycompress (headered ROM recommended) to extract and re-insert graphics to Yoshi's Island. YCompress is a Command Line tool, so to use YCompress, copy the Windows Command Line into the same directory as your YI ROM and YCompress.

You should be able to find the Windows Command Line by searching for cmd.exe on your Windows Computer. Then, run the Windows Command Line and type in the commands needed.

For extracting graphics, type ycompress 0 yirom.smc gfxfile.bin (note, that you have to replace the yirom.smc with the name of your YI ROM) and hit Enter. For inserting, use ycompress 1 120200 yirom.smc gfxfile.bin (again, replace yirom.smc with the name of your YI ROM and gfxfile.bin with the name of your GFX File).

Once you have a .bin file containing the graphics, you can edit them using a tile editor such as YY-CHR. Remember to re-insert the graphics after you edit them!

In YY-CHR, you'll need to change the graphic format to "4BPP SNES" for most graphics to appear correctly.

Ycompress normally overwrites all ROM data in the 0x120200-0x1F957F range when inserting, even though the graphics don't use all this space. If you type 11 instead of 1, Ycompress will only overwrite as much as necessary instead of filling the rest with 00s. If you don't understand what this means, you don't need to be concerned about it.

How can I edit the title screen logo?

Naulahauta has created a very comprehensive guide about editing the title screen logo. Note: the images in the tutorial are no longer available, but it is still followable.

If you don't feel like going through the complex process of editing the entire logo, you can still edit the "Super Mario World 2" and the "©1995 Nintendo" the same way you would any other graphics.

Is there a list of which palettes work well with each Layer 2 background?

This list contains images of each background, as well as various palettes that they function well with.

Is there a list of which palettes work well with each Layer 3 background?

There is here. It contains images of each background, as well as various palettes that they function well with.

Why does the sprite I inserted have glitched graphics?

Most sprites will only appear correctly in a certain sprite tileset (sprite set for short).

Is there a list of which sprites can be used in each sprite tileset?

Here is a spreadsheet by GbreezeSunset listing all the sprites available in each sprite set, as well as their names in Golden Egg and a preview of their graphics.

Yoshis Fan has also previously made a list of which sprites are compatible with each sprite set. You can download it here. Note: this document uses the names of sprites in EggVine v0.81+, so the sprite names listed here may not match up with those listed in Golden Egg.

None of the sprite sets contain the sprites I need. What can I do?

The sprite sets themselves are editable using this tool by Kipernal. It may not always be possible to create a working sprite set for any given collection of enemies, but it does add more possibilities.

Is it possible to change the Baby Bowser palette when the lights are on?

You can edit the palettes via SNES Palette Editor at the following addresses:

$3FEA00 - $3FEA1C: controls the first palette of the Baby Bowser room when the lights are on.
$3FEA1E - $3FEA3A: controls the second palette of the Baby Bowser room when the lights are on.