Hello
I was wondering if the images in the carts of for example pico-tennis can be changed for them being cooler or if that something you have to draw on the sprite editor

you can edit the cart label - it is under the

__label__

section.
usual way to tweak is to have a dedicated cart where you draw/import the label image (full spritesheet) and then copy the

__gfx__

section to the game cart.
note: must be done with an external text editor