A while back I did a tutorial thing called Doombunny using one of my current images at the time..
Today I plan to do something similar, altho not using one specific image but more using several of my images as examples and try to explain what I think is important in an image.

Keep in mind my favourite style of images is the non realistic and more painted look, most of the things I say won't be that great if you want a realistic feel to your images. And do keep in mind that any of the stuff I say below are my personal preferences/opinions and by no means "the right way" or "the only way" to do things :)

I suppose my 2 big loves when it comes to images is colour and lighting, if you have those down you've gotten far.. Obviously a good pose/setting/camera angle can be just as effective, but if the lighting is flat or there's just the wrong coloring then it usually doesn't help much..

Colours
So.. Colours.. Here there's usually two options that I tend to go for.. either quite strong, saturated colours or quite desaturated colours.. either way works if you do it right..

I try not to have too many different colours in an image and also try to make sure that the ones I do use harmonize with eachother..
Basically, there's alot to say about colour and this site does it way better than I can so I might aswell just link it :p

The Complementary color scheme is my personal favourite.
For instance here I've used a blue and an orange colour as my main colours, they're opposite of eachother in the colourwheel and will often make vibrant images.

Lighting
When it comes to lighting I really don't have that many tips.. I usually go by what my eyes tell me.. either something works, or it doesn't.. I'm sure there's sites out there that explains the technical stuff about lighting aswell, but sofar I've just gone by what I feel works for the image in question.
And fortunately even what I would consider bad lighting is fixeable in photoshop :)

Here for instance the render looks very flat and not very interesting, but by giving it a bit of a makeover in photoshop it all of a sudden becomes alot more appealing. (granted in hindsight I should have picked a different colour for her outfit, but I'm too lazy to go back and change it :P )

Another example of bad lighting.. I tried to fix it in this one but nothing seemed to work all that great so I ended up scrapping this one :p

And yet another example :)

Composition
I'm also a big fan of empty space in my images.. space can really set the mood of an image if used wisely.. especially if the mood you're looking for is somewhat melancholic.. also I try to rarely have the "main focus" in the center of an image.. it's usually much more "exciting" if it's more towards either side.. altho it usually depends on the image.. sometimes things work very well in the center aswell..
Another thing I find myself using often is tilted cameras.. these are all little simple things to make an image more interesting..

It's a bit the same as with lighting here tho, I don't really know anything technical about composition, I just go by what looks good to me. Don't be afraid to experiment! :)

Some more scraps that haven't really been finished for one reason or another

And finally I have an old image of mine as an example of what not to do :p

This is Gus.
I don't remember when I did this one but it was originally supposed to be a cover for a fantasy/RP magazine, but I ended up scrapping it and doing something completely different instead, thank god :P
Ok where to start..
The lighting is quite bad here, too many different colours that don't work together, the background really doesn't work and is quite distracting.. especially cause the texture on the wall is too "busy"...
basically my biggest problem with this one is that it's too busy in general and too flat in the lighting..