But I think that more outlining would help?
No, that'd be a very detrimental thing to do. Lineart != detail. Hard shadows and defined highlights, though..
Krava, I've got some more feedback for you on that last picture.
Red ghost skull is awesome. A little jaggy, but awesome. Smooth it , crop it out, save it.[/li] Human-esque face is.. Not as good. Very little definition, very little bone structure. [/li] Pay attention to the lighting. I've defined the angles of your highlights and shadows for you, and, well, have a look
. Right now, the forehead, nose, nose bridge and chin are lit from the top left, the upper head, cheeks and upper ear is lit from the top right, the jawline, bottom ear and back of the head lit from below. Next to that, the cast shadows go in every direction, look at the horn compared to the eye socket. Proper directional lighting is key to understanding structure from an image.
[/li] There's no hard lines, except on the grass-brush eyebrows. A few hard lines where definition is goes a long way. This also helps define area's more and bring out colour. The easiest way to do this is to work with a hard brush at a low opacity, just keep sampling to get smooth transitions. This way, the structure you're painting is a lot more visible, making it easier to read and easier to work with.[/li] Try and use more contrast! I've taken the liberty of desaturating the image, and there's very little definition left, as you can see here
. Use a more full range, with more contrasting light and dark area's to define structure. [/li][/list]The most important advice I could give you, though, is to keep checking your work throughout the process. For values, desaturate the image - or even start out in greyscale and colour it later. For basically everything else: Flip the canvas horizontally. This gives you a whole new look at your work, almost like a fresh pair of eyes. For going from a sketch to a finished piece, the main process is flip-fix-flip-fix-flip-fix.. You get the idea.
I must say you've improved a lot though - as mentioned, the ghost skull is fantastic. For the purpose of being used in Elements, you've also improved a lot on the scaling and background. The (sometimes harsh) critique I give is because I see that you're progressing quickly, and that's always worth a nudge in the right direction.