You can also dowload the SCI Companion and use that to rip all the assets from a Sierra SCI game which can be a useful tool for seeing how a game uses its assets. It will export a sprite sheet which you could then edit. Getting familiar with aseprite and how it works is probably also a worthwhile task for creating your own art assets as well. Just importing a sprite sheet is a little tricky but once you figure out how to do it it's very, very useful.