Which graphics software do these games use?

[sttormx]

Stellar Dream
S.H.E.L.T.E.R
Sunrise City
MILF's Plaza

Im totally new with game dev. Which graphics software do these games use?

 

[rayminator]

Stellar Dream
S.H.E.L.T.E.R
Sunrise City
MILF's Plaza

they are hand drawn by a artist