Hiya Green, I think they mean replace them in the folder rather than in the program. You can usually tell which is why by the names. They are in the shop/ccdata/images folder of your server (or you can export your shop to a folder on your computer and do it that way too. The files you want to replace start with the name smallmain...
I haven't done this yet on my server, but unless someone else comes up with an easier way, I was planning to do this by grabbing the largest images that SCC created in that folder, and using that image if it's decent looking enough and hasn't degraded much from the original if any. This way the files are already named correctly other than changing the beginning part of the name. You can use a program like EZThumbs to resize the pictures which also has a rename system built in so you can do both at the same time. This way you won't have to rename the files manually.
Hope that helps