Image viewer and comicbook ds require you to convert/pre render zoom states and thumbnails for your images since they have their own kind of browser thing to navigate.
Moonshell 2 will just list your images and generate the zoom states on the fly. So you have to wear that delay for each image and know your files by name unless you want to slowly track through them one at a time.
Image viewer can take a relatively small folder of nicely compressed jpeg images and turn it into a very bloated image viewer file. Drastically bloated.
Moonshell won't touch your original images so long as they're in a format it can read. It doesn't like certain jpeg formats for example.