You are not alone...http://stackoverflow.com/questions/15054512/error-when-hitting-run-in-eclipse-2d
(LOL--I was doing a search on alphamap.png and the above came up.)
BTW, questions about loading resources come up a LOT, as getting the addressing right, especially in jars. So, it doesn't surprise me that the problem comes up in an entirely separate context.
The Java Tutorials section that deals with resource loading is here:http://download.oracle.com/javase/tutorial/uiswing/components/icon.html
They are discussing this in the context of loading an image into an icon, but the part about loading images in general and about the relative addressing still pertain.
You might consider going through this set of Space Invaders tutorials instead:http://www.cokeandcode.com/main/tutorials/space-invaders-101/
There are four versions, and if I remember correctly, 104 uses LWJGL. I recall getting 3 of them to work--I remember skipping one of them.
I also wonder if the file is part of the Slick download, if there are some resources packed with the full distribution.http://slick.ninjacave.com/
The writer Kevin Glass is listed with JGO, and often comments/contributes. But since he officially handed this code off, he might not wish to be asked maintenance questions. I really don't know him well enough to say.