Phase one, THE CLEANUP For starter, you need an image, obviously, of relatively high resolution. Don't worry if it's a bit under 2048px, but don't take a highly compressed thumbnail. Also avoid all the already retouched ones, as they are often compressed, or burnt in the white and black. The best is still to get them at the source: the Nasa and Hubble websites. This is especially a good idea since all those images are NOT COPYRIGHTED.
http://www.nasa.gov/multimedia/imagegallery/index.html (http://www.nasa.gov/multimedia/imagegallery/index.html)
http://hubblesite.org/ (http://hubblesite.org/)
Since it's a game background, I'd suggest to avoid pictures that have large bright areas or too much contrast, as it might be too distracting.
(http://i.imgur.com/j2m5KLx.jpg)
It's a classic, true, but a nice one. Now we need to get rid of all those pesky stars. To do that, we will use the Clone stamp tool, and the Healing tool.
The
Clone Stamp allow the pick some area of the image (clicking with ALT) and then painting that area elsewhere. This allow to erase the big stars with nebula or space rather than with a solid color.
This is only useful to erase the biggest stars, the small ones will be taken care of later.
(http://i.imgur.com/y6VvoIq.jpg)
Now to erase the small ones there is truly a magic tool: the
Spot healing brush. After each stroke, the software will check the highlighted area for mismatched patterns or bright spots and erase them with surrounding details... It' like an automatic stamp tool that only erase small stars!
(http://i.imgur.com/ywz9zot.jpg)
In a few minutes you can get rid of most stars, don't bother whit the tiniest ones if they aren't too bright. A few Clone stamp strokes to erase the couple of spots the healing tool might have messed up (like on the borders) and you're good.
Phase two, THE EFFECT If your image wasn't at the right size before, this is the time to crop it to 2048x2048. Why now and not before? Well the healing brush does some artifacts, so the tinier they are, the better.
Now to get a somewhat painterly effect: You may have noticed before, most of the "painting" effect in photoshop don't work that well... However there is one unrelated effect that, if overused, give a nice airbrush aspect. That effect is the
Reduce Noise. The settings are simple: max strength, no sharpness! Generally you need to use it between 6 and 10 times to get a nice smooth effect. Also keep a copy of your layer after a couple of effects, for later use.
(http://i.imgur.com/rnTzadf.jpg)
A second useful effect is the
High Pass. This effect will allow you to give back some local contrast to your image (meaning contrast between shapes, without crushing your whites and blacks). Duplicate your less denoised layer several times (I personally go to 4) and apply the high pass effect to each of them, each time with a different precision (in my case, 2 pixels, 5px, 10px, 20px). After that you can combine them in Overlay fusion mode. Why to use that cumbersome method instead of applying a simple sharpness effect? 2 reasons: much less artifacts, and more control since you will be able to change it's opacity or paint a mask later.
(http://i.imgur.com/biWg6Yf.jpg)
Then all is left to do is combine those layers: The smooth one at the bottom, the sharper one above in Multiply fusion mode and 25% to 75% opacity, then the High pass on top in Overlay mode and 25% to 75% opacity.
(http://i.imgur.com/MHU66iG.jpg)
Phase three, THE COLORS This is the fun part: curves, color correction, hue saturation, gradients, layer in Color fusion mode, everything is good to change the color of the nebula to your liking. As a finishing touch, I sometime add an different image on top in Color mode after blurring it. It add some nice variations in the color that might be missing when using too many gradients and color corrections.
(http://i.imgur.com/0ifGt6o.jpg)
Don't forget that too much contrast and saturation can be distracting when playing!
Phase four, THE STARS This can be a tricky part. You may have noticed, it is actually quite difficult to find a nice starry background on the internet. Most of the time there is too much stars, not enough, too bright, too small, with a nasty star-glow or even a lens-flare! Things are even more complicated by the fact that the games background use a very stylized aesthetic (slightly blurred, often not completely white in the center). Of course the best course would be to take a brush and paint your own stars, but there already is a starry background in the game. So most of the time I don't bother and directly integrate the background_4 in mine (the subtle nebula in it often add a nice effect) that way I'm sure to stay close to vanilla.
(http://i.imgur.com/dE0LMV3.jpg)
You'll notice there are more tiny stars behind the nebula rather than in front for obvious reasons.
After some finishing retakes, I get this:
Spoiler
(http://i.imgur.com/JW4SCPh.jpg)