The image size must be power of 2. Only a few graphics cards can deal with non power of 2 textures. The texture containing your 600x405 image must therefore be of size atleast 1024x512. This doesn't neccesarly mean a lot of waisted space as you can put many images in that texture.

I would also recommend DevIL instead of javas builtin image capabilities. DevIL has much more functionallity and is (in my opinion) easier to use.

what is devil mate and where can i find it, do i then just simply type in which texture to use using that?
thanks

DevIL is an open source image library. You can find more information at http://openil.sourceforge.net/

Unfortunately you can't just supply a filename and expect a texture id in return, but it's almost as easy. On NeHe's site, you should be able to find his tutorials ported to Java and DevIL.

LWJGLs API doc does also contain some information about DevIL.