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The GIMP it's a powerful tool but you don't need to be an expert to make some nice textures for your games. I'll show you how to open an image and do some basic manipulations that will permit to obtain (i hope) a fairly good work.
Sorry if some names will be imprecise (for English version users) but my GIMP "speaks" French.
Of course, the techniques explained here, will be useful for almost all the others image manipulation programs ( ex: Photoshop, Paint Shop Pro, Corel Draw, etc...) with some twists, so, "get in"!!
Let's start. Open the GIMP, and in the file menu of the toolbar, open the screenshot image you have done with Blender (or other one of yours), and, with the guidelines (take the cross-arrows/move tool and drag the mouse at the left side or top window border to "get" the guidelines) draw a square over the darker grey area border. Be precise.
After this, with the "cut" tool (red square in the image above) drag the mouse over the guidelines square (enable "snap to the guidelines" in the View menu - right click in the image window and put the mouse cursor over the View item).
Check in the settings window if the "cut" selection is really a square area (ex 256x256).
Now press the "cut" button.
Right click over the image and choose "Image-Size of the image" in the menu. Scale the image at 1024x1024 (it's better to work on a bigger image, for details, and scale it down later).
Right-mouse click, and in the Selection menu choose By color, and left-mouse click in the grey zone (keep the default values). Press the Reverse button to select the white lines.
Save your work frequently, in the GIMP native format .XCF (to keep the layers, chanels and paths).
Do a right-click over the image window, and in the Dialogs item choose Layers, Chanels, Paths and Brushes, to bring those windows to the scene (if not done yet). With the white lines still selected, Ctrl-C to copy the selection, and Ctrl-V to paste it.
This will create a new layer with a floating selection ( 1).
With a big brush size "paint" the lines (drag the mouse over all the image) with a Black colour.
When done double left-click over the new layer thumbnail in the Layers window and in the pop-up window enter an useful name and Validate ( 2).
Now you have a "real "layer with the black lines and a transparent background ( 3).
You can now, disable the Eye icon of the Background layer, we don't need it anymore.
Now, the true texture work can begin. But...many, many pages would be necessary to show all the techniques and methods. Anyway, I'll show you some of them I use.
Add a new "white" layer, place it below the "lines" layer ( in the Layers window), and colourise it with your future texture "general" colour
(here, a grey shade), with the Fill tool.
An useful technique, often used in texture work, it's the Bump mapping (or relief). Let's do it.
Add a new transparent layer above the grey layer, and draw some Black lines inside
the "guide lines". Apply a Gaussian Blur filter to them. Now select the grey layer
(click over his thumbnaill in the Layers window, to make it active/blue), right-click, and in the Filters item choose Render-Bump Mapping?, and you'll get the same window that's in the image at above right. Use the same settings (more or less), check the result in the preview window, and when you're satisfied, press Apply. You can hide now the Black lines layer.
It's useful to have some textures and motifs libraries in your hard drive to add to your image (and speed the work). Load one in the GIMP and bring it to your image. Doing that in the GIMP is a bit different than the other programs. With the mouse in the loaded image window (active) do Ctrl-C (copied to the buffer), activate your texture image window, and do Ctrl-V to paste the image.
This will create a new Floating Selection layer that you must double-click in the Layers window to apply.
Place it above the grey layer and with the Move tool (double-cross) place the image acording to your desire (normally above the bump work).
This new image hides the grey layer bump mapp, but it's necessary to use this very interesting technique.
In the Layers window with the new layer selected, click in the Mode box and "play" with the different modes until you find the "good" one (image at below right).
You can erase all the image parts outside the "guide lines".
Keep in mind that adjacent edges ( the ones with same colour points) texture, must coincide in order to seamless texturise your BLENDER model. The Clone tool can help here, or, select little areas and copy and paste them. When you've decided that the work is finished, save again in .XCF format, right-click and in the menu items, choose Layers-Flaten Image, this will remove all the image layers and make it exportable for other image formats like JPG.
When done, right-click, in the menu choose Image-Scale image and fill the size fields with your desired final texture size. Then right-click, File-Save as and choose the JPEG 1,0 compression.
If you need a texture with transparent zones, add a transparent layer, place it above the old background layer (image at left), righ-click and in the Layers item choose Flatten visible layers?. Now, that you have an image like the one above righ (with visible little chess squares), scale and save it in TGA format WITHOUT compression.