I used KeyEvent and MouseEvent from java awt/swing. Is there an easy way to convert those classes over to the input methods in Slick?
Probably not; but then again, refactoring a game to use a different library is never easy. You would probably have a better time restarting from scratch; it would also allow you to fix up your coding style and tailor your game exactly to Slick2D, rather than trying to force a Java2D-style down OpenGL's throat.
All of my graphics are in a sqlite database, and I use ImageIO to read the data in from blob fields directly on start up. Would I still be able to load my images directly into Slick from the database.
OpenGL games should use a texture atlas. Whether you decode your images on the fly into a large texture atlas at runtime, or whether you do something else to pack your textures together, you won't get very good performance without a texture atlas.
Is there a way to load an Slick Image from a BufferedImage? I could the canvas into different "viewports" and made my own little windowing system.
Yes but you should not need to do this. Ideally, your Slick game should not have any dependencies on AWT. For offscreen rendering you should use Image.createOffscreenGraphics (part of the dev branch) and getGraphics().
How do I lighten and darken an image in Slick?
With shaders or fixed-function glTexEnv described here:https://github.com/mattdesl/lwjgl-basic ... &-Contrast
(this is for LibGDX, but similar concepts apply)
How do I blend an image onto the canvas so it is like 50% translucent?
image.draw(x, y, width, height, colorWithAlpha);
Does Slick2D play ogg files?