Each volume of Made in Abyss has a different character on the printed copy's spine, and these illustrations can be found on a glossy color page in the manga... save for the first three volumes, which may have been on a separate manga cover?
I've cleaned them up and erased the background elements, just in case you need some cute MiA chibi PNGs!
The first three volumes didn't have a digital version available, so I had to do a bit of smoothing and color correction in Clip Studio Paint. I did my best, yet they're a bit lower quality than the digitals.
Thanks to Onyx for sending me a good quality scan of the Volume 3 Nanachi! 💜