Probably due to your orthographic matrix or texture coordinates, as Heroes said. Also; if you are using a non-power-of-two texture and full texture coordinates (i.e. 1.0) then it might lead to the texture being rendered incorrectly. To fix; use correct texture coordinates, a non-power-of-two texture, or a better texture loader (SlickUtil is outdated).
Hate to say it, but the Space Invaders tutorial is pretty old. It uses "immediate mode" (glBegin/glEnd) which is deprecated, and teaches some poor practices. IMHO if you want to learn OpenGL, you should work with the programmable pipeline. There is more info here:https://github.com/mattdesl/lwjgl-basics/wiki
Specifically it would be helpful to learn about GL initialization, textures, and texture coordinates in OpenGL:https://github.com/mattdesl/lwjgl-basics/wiki/Displayhttps://github.com/mattdesl/lwjgl-basics/wiki/Textures
Using a library like lwjgl-basics
would make sprite rendering a lot more efficient and a lot easier:
spriteBatch.draw(myTex, x, y);
spriteBatch.draw(myTexSprite, x2, y2, width, height);
LibGDX is an even more powerful library that I can't recommend highly enough.