[feature request] Guide on custom C-Gear/Pokedexes?

[Camwoodstock]

Currently, the instructions provided for how to upload custom images for the C-Gear/Pokedex are... well, uh, basically non-existent. The only guides I can find online are for doing so via PKHex and another program called CGearpoke. The feature is there on Entralinked, and I am very curious about it! But I have no idea how to use it. Documentation is always handy, but it'd be especially useful for these, I'd wager.

[kuroppoi]

It should work the same, though the end result might be slightly different.
The problem is that the format in which skins have to be stored is quite finicky and doesn't allow for a lot of freedom, and images typically either just happen to work or have to be created specifically to work as C-Gear or Pokédex skins.
The limitations aren't necessarily complex and once you understand how they work it should become relatively easy to create custom skins. Images must have a size of 256x192 pixels (which is the pixel resolution of a DS screen) and cannot contain more than 16 unique colors.
C-Gear skins have an additional limitation, that being that they cannot have more than 255 unique "tiles."
A tile is simply an 8x8 pixel section of the image aligned in a grid, and these tiles can be flipped and re-used in other parts of the image in order to save space.

Take a look at this Munna skin for example; it appears to be relatively complex, but when you draw a grid on top of it you will see that a lot of tiles are identical and can be reused many times.

Interestingly, Pokédex skins do not have this limitation and allow you to use all 768 tiles that can fit on a DS screen, meaning you only need to worry about the image size and number of colors.
But yeah, these technical limitations make it a bit tedious to create custom skins. The converter will automatically detect duplicate tiles and reuse them, but it's still the user's responsibility to manage the number of colors and overall tile complexity of the image.
I believe that some image editors let you reduce the number of colors to a set amount, so that might be worth looking into.

For now, I recommend playing around with the custom skin upload feature to see what's possible.
Make sure to preview your skins after uploading them (especially Pokédex skins) as the format might cause slight changes to occur in the image and the preview feature provides an accurate representation of what it will look like in-game.