ive always used the depack file from here
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Just copy it and place it in the folder than has the "image" or "archive".rpa then run the game and it'll create an "unpacked" folder that will fill up with the ren'py game art. if you want to run the game normally without it taking the art just delete it and the other depack thing it creates.
Lots of games now have the art as webm so I use something called XNconvert to change them to .png. Depending on the game that can take a while so its best not to convert gigs and gigs in one go.