Use your command prompt or file browser to copy a file to the desired
location.

Right click on the Source Packages node, choose New->Other.

In the wizard, scroll down the tree on the left, choose the Other node,
then in the box on the right, scroll down to find "Empty File." Name it
whatever you like (get the extension correct).  This will add a file to
the root of the project.  You can now edit the file normally (ie., fire
up Gimp and edit your PNG file, or replace it with the image you want).

To add a file to a package, do the same, but right click on the package
node instead of the Source Packages node.

As Sabine mentioned, you can also just dump files directly in your
source package directories.  After you use the method I gave above a few
times and you see where the files actually go, you'll get a much clearer
picture how to do that.  It's easier than going through the NetBeans
menus for most things.

Remember that you can switch to the File view of your project if you
aren't sure about the path of any given file.  You can also right click
on a PNG file, choose Properties, and the path will be listed there,
without having to switch views.