The essential things in sigs are
lighting, flow, focus, and depth. A disclaimer: the things I will talk about in this guide are stuff that I personally learned. I am in no way saying it is absolutely correct/without fault or the only way of doing it. If you have something you disagree about, feel free to contact me about it
Lighting: You need a light source in most of your sigs. Lighting should generally be close to your render/focus and can also help determine what the focus is. You want it to be visible, yet not overpowering. In real life, if someone takes a picture, most of that light will generally come from the sun. However, in sigs, you can place your light source anywhere. The upper corners of your sig are easiest(IMO). You can also put it at the bottom of the render to make it look like the light is shooting up. Some ways to do it(on Gimp) is to do a white(or whatever color you choose) to transparent gradient and set the shape on radial(circle-like). Or, use a soft brush and then gaussian blur it by 200 or so and it'll be good. Careful though, because if you cover your render partially with a blurred light source, it can make it seem like your render is LQ. Here is a
good example of lighting.
Flow: Flow is pretty much the motion in a signature. It's where your eyes go when you first look at a signature. Now, you may think that only gifs have motion, but static sigs can too. Take my
toshiro sig for example. Pay close attention to how I smudged. The flow is obviously from the bottom to the top right. Flow can be used nicely to compliment the flow of some of your renders. Don't know what I mean? Take a look at
this render. See the direction that his cape is blowing? That is an example of flow in renders. With this render, you can smudge the background to go in the same way as his cape and create good flow in your signature.
Focus: Focus is what you want people to first see or see the most when they look at your sig. Most of the time, it is the render or a specific part of it. Of course, people will see all the other details, but your render/focus is the most important, not the text or the other stuff in the background. The positioning of your render can have a significant impact on your focus. In art, there exists the
rule of thirds. What this basically states is that if you divide a piece of art into three parts equally vertically and horizontally, where the lines cross is where people focus on when they first look at a piece of art. Focus can also be created by creating depth(explained below) and sharpening the render. Don't sharpen too much though, because then the render will look weird like
this. Be very careful when putting stuff over your focus. If done incorrectly, it can make the render/focus seem LQ, when it isn't in actuality. However, this isn't to say you shouldn't put stuff over you render, because this is often a helpful way to blend.
Depth: You know how when you focus on a specific object the rest of the things in your vision seem blurry until you focus on it? That's part of the definition of depth. Depth being able to see that something is closer than another. In a sig, you want to make sure that you can differentiate what is in front of what. Take
this for example(the one with the girl Nu). I blurred the background and sharpened Nu a bit. See how you can obviously tell that Nu is closer than the bg? If you want to see the difference, look at
sig 1 and
sig 2. The first is with the background normal and the second one is blurred. Notice the difference? Also, as I stated above, this creates focus because you automatically focus on the render, and not the bg. If your sigs don't have depth, then they will look like when people try to take pictures of themselves taller than buildings and stuff.
Blending: Blending is the technique of making your render look like it fits in your signature. Look at
signature 4 and
signature 5. The first one looks a lot worse than the second one right? That's because it looks like the the render was just slapped onto a background that I made. The second one looks a lot more natural. On the second signature, I took the time and effort to blend my render into the rest of the signature. In short, blending makes your signatures look much much better. The best technique that I have found for blending so far is smudging. That is just me personally, though; you may find a lot of other good ways to do it.