Hello, i'm a no-code guy enjoying game design tools.
My humble take on what you say you don't understand:
1. is image resolution the secret noone will tell you?
I don't think so, because you created assets with multiples of 8.
It doesn't matter if your original picture is square. You need to adapt it to how it will be rendered.
2. is it that Bitsy has a stable number of pixels?
As a javascript blob (from what i understand), Bitsy offers a gamedesign experience which is necessarily compact.
Here the sounds and images and canvas size are minimastically designed on purpose.
(to make it work first i guess? or to save RAM space and make it work online?)
So in this Bitsy engine, picture size is fixed, 8x8 for all, sprites n' avatar n' stuff.
...Maybe you'd like to find a double-down alternative here. (Bitsy HD) https://vonbednar.itch.io/bitsy-x2
I really wonder how you managed to import assets in Bitsy.
Anyway, yeah, you're going to HAVE to have them be smaller anyways.
Or redraw it. The pixsy solution mentioned above is good for rooms but can't upload sprites.
Sorry for your potential art loss.
Hope this helps.