1. The idea here is to create your own template and have it detect something that you captured with your camera (immediately convert to monochrome). Note that we a bit constrained here because we have only 5x5 templates in GIMP. So, find an object with a simple shape. It might be something mostly light or mostly dark. Take a picture of it against the opposite background (light on dark or dark on light). A piece of paper can provide a find light background. Make sure that the object is somewhat small within the image. The idea is that when you shrink the image, the object will be end up being at most 5 pixels across. Note that you can use any stuff like histogram equalization or gaussian on your images if you so desire
After shrinking the image to say 100x100 or 50x50 (whatever it takes to make the object 5 pixels across), inspect closely the pixels of the object within the image. Try to come up with a mask that will "light up" or "darken out" whichever the case may be when it is perfectly placed over the object. Now, using the mask that you created, convolve the entire image. Your object should stand out in the result.