How To Create A Postcard in GIMP

Step 3: Castle - Increase Highlights And Contrast

Make sure that the Castle layer is selected. Open the Curves tool (Color / Curves... menu).
I labeled the highlight (white) point with 1 (upper-left corner) and the shadow (black) point with 2. This time there is no guide to follow, all counts on your own feeling. The decisions I've made for this tutorial are not necessary the best and may not work with your image. Once again, experiment until satisfied.

Click on point 2 and drag it to the right until meets the histogram.
Click on the curves line anywhere between point 1 and 2. This will result in point 3. Drag it to the indicated position. Due to this action, the highlights will be deeply increased. Do not worry about the loose in detail over the sky. We will take care of this at the next step.
Click anywhere on the curve, between 2 and 3 and drag to the indicated position for point 4.
Click anywhere on the curve between 2 and 4 and drag to the indicated position for point 5.
By inserting and dragging points 4 and 5 we created a 'S' curve which results in an increased contrast on the layer.

After these actions the image looks like this (click to enlarge):