The easiest way to import textures and sound is by using the import wizards. They will copy the asset files to the correct folders and produce all the necessary NoveltyML for you.
Simply click the Add files button and select the files you wish to import from anywhere on your harddrive.
Once the assets have been added you have the option to configure how they should be imported. You can set a new name and a filter (folder in asset library). You can single files or multiple at once.
When you're done, you can either import the assets to an XML file or a content package. A content package is a single file and thus very easy to distribute.
For textures some formats, such as BMP, will be automatically converted to something smaller in size. GIF animations will be converted to PNG or JPEG and an animation asset will be generated.