you do not need an app for wallpapers. any images you find as long as they are in jpeg format should work. you may need to use a photo editor like photoshop or paint(free) to resize the image.
as far as file manager goes, i use root explorer. not only can it read and write, but, it can do a search for files on the system partitions as well. the search feature is very handy as devs tend to hide where the bootanimation file is hidden. another good one is es file manager, but that one you can't do a search in the system partitions like you can with root explorer.