I believe it is easier to create a .mym using ThemeMii, where you can add, for example, a simg, and then browse in the base .app for the image you want to replace, preview it to check it is what you want to replace, then add the file to replace it with, and it automatically does the containers and I think also the size of the image.
EDIT: Yeah, I just tested this, it is as I said. Incredibly easy. You can find whatever you want to replace and replace it, very simple. Manages image size (from PNG or TPL, your choice) and containers and also source (i.e. you tell it the file on your computer, it'll put that in a folder in the mym and auto-add the source to the ini). Much easier than doing it all manually IMO.