Can I have nested folders for images?

Yes, you can nest images.

For example, if you create a folder called images in your book's resources folder, then you would type something like this in your book:

You can nest images further if you like. So, if you nested a sub-folder in in your images  folder, which is in your resources  folder, you could type something like this in your book:

You can name the subfolder whatever you like.