If you don't want to download a whole cg pack just download
You must be registered to see the links
Copy and paste the depack.rpy file in Summer time sagas game folder and run the game. after a while (depending on pc speed) a folder called "unpacked" will appear an start filling up with the art. After it's done just delete the depack.rpy file and the newly created depack.rpyc file so you can play the game normally in future. The depack file will work on any game made with Ren'py.
For summer time saga you will need to convert the images, i use
You must be registered to see the links
you can select multiple files and they will all convert at once.
Then just use GIMP to muck about with them.