Hello!
I initially was toying around in unity before making it to MakeCode- there is functionality within unity to split your sprite sheets in the editor- i haven’t been able to find a way to do so and curious how MakeCode programmers handle it or if that functionality exists already.
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Sadly – from what I know – there isn’t. 
Closest thing would be to convert a spritesheet to makecode using this converter, then splitting the sprites manually by selecting each sprite and copying it over to a new separate image. 