All right, before you decide to make kgpanels artwork yourself, there are certain things that you have to know about kgpanels itself. Most importantly, any kgpanels graphic file must be saved as targa file (tga) and can only be saved in specific pixel sizes. Namely, it must be powers of 2 - so 2, 4, 8, 16, 32, 64, 128, 256 or 512 in width and height. You cannot exceed 512 pixels on either width or height of your image, but you can drag the layer in game to suit your needs.
You can however freely choose the size on either side - ie it doesn't have to match - as long as you stick to powers of two.
Now, it's time to introduce you to a very convenient photoshop tool - brushes. Brushes are going to be your main source for blending amazing backgrounds together. You can either make your own, which would be enough content to fill yet another tutorial, or get them online. Places such as deviantart are a good source for brush sets. Download whichever you like and put them in the brushes folder (path depends on your photoshop version and OS, mine is C:\Program Files (x86)\AdobePhotoshopCS4\Data\PhotoshopCS4\AppData\ Adobe Photoshop CS4\Presets\Brushes).
Once you find and install some sets you like, it's time to get started. Always restart Photoshop after installing new brushes to have them show up. So, open a new file, choose the size based on the aforementioned rules and what dimensions you want your piece to fit and make sure your color swatches have the default colors selected (black and white).
Having the background layer selected, go to Filter > Render > Clouds and hit Control + F until you have a more or less even layer of clouds without black or white dominant.
Create a new layer and change its blending mode to "Overlay".
Then select your brush tool, click on the arrow next to the brush, then click on the arrow on the window that opens up and your brush list will extend.
Select a set of decently sized brushes that make for a good background effect and start applying them. Alternate between black and white foreground color as you do so.
When you're happy with the look of the layer, make another layer on top of it and repeat the process.
Apply as many brushes as you like, but make sure to create new layers with blending mode set to overlay as you go.
Once you have a background you're satisfied with, you should apply some more specific brushes to give your layout a sort of theme. Or alternatively, if you want to keep it a mute background, apply scanlines or something of the sort to dim it down.
Once you're happy with the overall feel of the piece, it's time to apply colors! Go back to your very first layer, the cloud one, hit Control + B and adjust the sliders until you come up with a color you're comfortable with.
Repeat this step with every layer until you're happy with the outcome.
When you're done, you can add a border if you wish, by making a new layer, choosing the rectangular marquee tool, selecting the entire piece, right-clicking it and picking "stroke". Width should be 1-3 pixels and color neutral. Black or white or grey are usually best.
If you're a little photoshop-savvy you can at this point apply further techniques and blending modes such as blurring or sharpening parts of your image or applying design tricks to make the image look more functional.