1. You need black and white (not grayscale) images. This is easily solved with a tool like HyperDither, which uses “dithering” to create shades of gray using only black and white pixels.
For example, changes this color Bulbasaur into this black and white one:

This is nice. HyperDither has a batch mode, which works, but it actually makes slightly smaller PNGs if you do it one at a time.

2. Saving space. You get 24kb RAM per app and 96kb storage for images/fonts per app. It’s not easy to fit all the images you need in 96kb. I wanted 60 images, so I had to average ~1.5kb per image. Luckily, black/white 144×144 PNGs are not so big. Using HyperDither, I got pretty close to 96kb, but not quite: I’d have 90kb of PNGs locally, but when I uploaded them to the Pebble, it converted them to “pbi” files, which are almost double the size.

(also note that if you include too many pictures that are over 96kb, you’ll get a cryptic error: it’ll compile fine, but then will say “Installation failed. Check your phone for details.” but no real way to check your phone for details, and no hints about why it fails. You’ll get a similar cryptic error if you upload an image but never reference it in your code (stack overflow related question).

Spriting
So I figured there was some overhead per-image, so I tried spriting, or combining multiple images into one big image, like so: (created with InstantSprite, which is awesome and free)

and then you just take a subset of that image at a time. Pebble provides a call,gbitmap_create_as_sub_bitmap, to do exactly that. However, a few problems:
– if you make an image that is too big (like 10 pokemon at a time, or 1440×144), then gbitmap_create_as_sub_bitmap just crashes the app with no feedback. I found that 6 pokemon (864×144) worked, while 1440×144 didn’t. Not sure what the actual limit is.
– it doesn’t even make the images smaller! 10 PNGs with 6 pokemon each is not really any smaller than 60 PNGs with 1 pokemon each.

uPNG. Forget Spriting.
So, forget spriting. I then found Matthew Hungerford’s port of uPNG, which is a library that lets you use raw PNG files instead of converting them to PBI first. Just include the .c and .h files, use gbitmap_create_with_png_resource instead of gbitmap_create_with_resource, and then edit your appinfo.json file to change the type of each image from “png” to “raw”.
(editing your appinfo.json may require pushing your code from CloudPebble to Github, then cloning it locally, editing the file, committing and pushing your change. also, you might have to move your image files from resources/images/foo.png to resources/data/foo.png. anyway, this is nice, because you can then upload a bunch of images by editing a text file instead of uploading through the GUI a lot.)

He also provides scripts to transform images to B/W or dithered first, which ended up saving a few kb in my case.

In the end, thanks to uPNG, I had 80kb of images that actually stayed 80kb. (plus a little bit of overhead per image, but didn’t matter.) Success! Here’s my watch code on Github.

Other Tips
– Use logging, like: APP_LOG(APP_LOG_LEVEL_DEBUG, “hello”); – this took me too long to discover.