Thank you for your feedback. We have thought about adding this kind of functionality but we're not sure if it will work out well because of the way images are stored in mojoPortal. All images are just files and mojoPortal does nothing to the images when they are uploaded, like create thumbnails and other sizes. So, when browsing a directory, each image would need to download for the browser to display it. If you open a folder with a lot of images in it, the file manager could slow down dramatically.
We haven't decided against doing this but we are trying to figure out how to do it in a way that doesn't slow the file manager down a lot.