Try using stock images at different layer modes and opacity levels for a better blending between the "render" and the background, one of my favs is 4 stock images: 2 stocks "soft light" mode and whatever opacity looks best, and two stock "normal" with parts of it erased to fit the focal, along with opacity changes as needed. You will have to play around with the layer position of the stocks because it is different for each design.
If you had photoshop I could show you a few really good blending techniques but I am not very familiar with gimp.
Always Always save a PSD or Stage file so that you can make changes after getting second opinions or if someone wants something changed. Overall I have seen significant improvement from the beginning of your app to now.
Try and google different tutorials. This will help you to step outside your comfort zone and learn new techniques. Great job though. Keep up the good work!