In the .zip you have a .rpy file which you can put in your "game" folder. when you then start the game it will save every image ingame as a .png picture into a new folder named "unpacked". I also put a manual to the file.
For me the .png pictures are kinda buggy so in convert them with XnConverter from .png into .png (yeah i know...) and then they work perfectly fine. maybe you dont have to do that just try it.
For working on fanart i only work with paint.net (not paint!!!). its pretty easy to use and for free. i think photoshop is kinda complicated.
Work with different layers and if you put bodyparts from different characters together change the color settings so they match. then put the layers together and blurr the areas where the bodyparts connect.
If you have any questions just ask