That would be because you are supposed to use the files you downloaded from that post. The text at the bottom under "IMPORTANT!" is clear about how to tell what you are replacing.

I think you should read the instructions again. You don't have to extract the files from the game; you can use the ones provided in the post. It is easy to tell which painting will be replaced, as the second set of images (which are shots of the original Maxis paintings) correspond in order to the replacements Sara made. Also, she kept the file names the same as the original extracted paintings.

Say you wanted to replace the painting of the tiger. You open up the zip file she's provided and find the file titled REPLACE_painting_tiger.package, and open that up in SimPE. It then goes from step three onwards.