Bullet holes are very easy to make using GIMP. Before I can get you started on this tutorial, you'll need to check and see if you have the Layer Effects script installed under Script-Fu on the menu bar. (See image below.)
The Layer Effects under FX-Foundry won't work in this case (I've already tried it, unsuccessfully.) So you will need to go to the GIMP Registry site and download the correct version of this script, found here.
To save and use scripts for GIMP (*.scm files), most users open the files using Windows notepad. Keep in mind that by using Windows notepad will require you to save the file using a different tactic. If you are using notepad to open *.scm files, when you save them to GIMP, be sure to change the "Save As Type" to "All Files". And for extra measure, in the file name text box, add .scm to the end of the file name. In this case, the file name will be "layerfx.scm, and that is all you should see in the filename text box. The file should be saved to C:\Program Files\GIMP-2.0\share\gimp\2.0\scripts\
Once you have saved the script, you'll need to check to see if its loaded and then test it. Open GIMP and check under Script-Fu on the menu bar. Do you see Layer Effects listed? If you do, open a new drawing (any size). If you don't, check to see if "layerfx.scm" is in your scripts folder (see directory string above).
The Layer Effects require that any selected image be on a "layer". So, add a new layer to the drawing. Use the Free Select tool (Lasso) to draw out a selection, then visit Script-Fu - Layer Effects on the menu bar. Are the Layer Effect options greyed out? (Double check that you have a layer selected or highlighted). If the options are available, you're in business.
Open a new drawing 360 x 360, white background. Add a new layer.
With the new layer highlighted, select the Ellipse Select tool and tick "Expand from Center" and "Fixed: Aspect Ratio" in the tool options. Drag out a circle about 115 x 115 pixels in the center of the drawing.
Leave the circle selected with the "marching ants" and select the Free Select tool (Lasso), Start drawing out a "star shaped" area around the circle, like shown:
It's not perfect, but the idea is give it a "shattered" look. Close the selection but clicking the first point you started with.
Set the foreground color to a mid gray color, select the Airbrush tool and paint the entire "star shaped" area. Leave the area selected with the "marching ants".
Remember the Layer Effects mentioned above? We're going to use those effects now to give this star shaped area a blasted paint look, mimicking a cut-out dropped shadow. Go to Script-Fu - Layer Effects - Bevel and Emboss on the menu bar.
Follow the settings as shown:
It should now look like this:
Add a new layer.
Select the Ellipse Select tool and drag a circle out about 70 x 70 pixels in size. Center it in the gray star shaped area.
Select the Airbrush and using the same mid gray color, paint in the circle on the new layer.
Go to Script-Fu - Layer Effects - Bevel and Emboss, and enter the settings as shown:
You should get something that resembles this:
Add a new layer. Using the Ellipse Select tool again, drag out a small circle and center in the middle of the embossed area. You might be asking, "Why not just just the Airbrush tool and paint in the circle with a circle brush?" The answer: because using the Ellipse Select tool allows you to set feathering at "x" amount of pixels, leaving a nicely faded or softened circular area, which is the look we're going for here.
With the Airbrush tool, paint in the circle with black. It should look like this:
Now all that is required is setting your Airbrush brush to a very small setting, with foreground set to black, and then draw in small "cracks" or "splits" around the black circle. You're finished result should look like this:
If you think the cracks and splits are too crisp or solid looking as you edit them, you can always soften the look using the Blur/Sharpen tool.