Couldn't find a good photograph of mine to use (I am not photogenic), so I used Daniel Craig :D. This tutorial shows how to put a photograph on a hoarding. Nothing complex, just a simple thing. It just came across my mind, so I thought to write a tutorial.
GIMP version - 2.6.4
Image size - hoarding (1024x768), Casino royale poster(799x403)
Here are the two images that I started with
..this is what I ended up with
Following is what I did:
1] Open up the hoarding image in GIMP. Now, before doing anything else we need to make sure that the image that we are going to put on the hoarding is in (almost) same aspect ratio as the hoarding. To get the aspect ratio of the hoarding use the 'Measure Tool' to get the height and width of the hoarding, as shown below:
For the images I used the aspect ratio came out 2:1 which almost same as the image, so I didn't have to crop the image. If the ratio is different, make sure you crop the image before putting it on the hoarding.
2] Open the second image as layer. I'll call this layer as "poster layer". To put the image on the hoarding, use the 'Perspective Tool'. Reduce the poster layer opacity and in the perspective options select 'Preview' as 'Outline', this is not necessary to do, but it will help.
3] This is what I got after applying the perspective transformation.
4] Now, we need to do some clean-up stuff. As you can see the corners of the hoarding are round, there are few cables infront of the hoarding, a light at the bottom. Add a layer mask to the poster layer. Reduce the layer opacity so that we can see the hoarding border through it. Select the layer mask and paint black color over the area that we don't want to be visible. You can zoom in to do it neatly
5] This is what I got after rounding the corners:
6] Now, the cables: hide the poster layer, select path tool to create a path over the cables. Select the layer mask and "stroke path" with black color. I guess most of the cables in front of any such hoarding should be easy to handle.
Similarly, do the same thing for other cables. This is what I got:
7] Almost done, just the final touch. The colors in the poster look too bright compared to the surroundings. Reduce the brightness and contrast a little for the poster layer.
8] And then, here is the final result