I thought I'd share what goes into making a banner image like the one I'm currently using at this web site. This is as much so I'll remember how to do it next time as that I think anyone will find it useful, but that's reason enough :)
The first thing is that my tool of choice for this is Paint.Net. Paint.Net is freely available, so go download it if you don't already have it.
Next, you need to find out what size (in pixels) the image needs to be. The image used here is 958x102 pixels.
After that, pick out what pictures you want to include in the banner and put a copy of each in a convenient place.
Now what I do is create a new blank image with the canvas size set to my desired banner size (again: 958x102 here). Set the background to be white. Then open each image image you want to use in turn. When you open an image, select the area you want to use for the banner and crop it down to that area. It helps to leave a little extra to the right and left of the portion you want to see. After that, resize each cropped image so that it's height matches that of your banner (again: 102 pixels here). Keep the aspect ratio or it will look weird.
When I'm done with all that, I'll end up with several image thumbnails in the top right of the Paint.Net application, along with the blank background. The next step is to decide on layout order. When choosing the order, one thing to consider is that you want to put similar colored images next to each other to make blending easier. For example, here I placed an image with leaves next to an image with grass, and an image with snow next to a brightly-lit store. If you need space for a title, as I did, make sure you pick an image that will work well with your title for that area.
Now it comes time to start placing the images onto the background. I want to leave a little space at the left side of the image so that I won't have too much noise behind the web site title. This means I have to start on the right-hand side and work my way left rather than the other way around, even though left to right will seem more natural for most people. For each image, copy the image in Paint.Net and paste it into a new layer over your background. Carefully drag the image to it's place, remembering to overlap the previous image enough to be able to blend them together. Hit the escape key to clear the selection rectangle.
Now it is time to use the gradient tool. Make sure the tool is in transparency mode, and very carefully click and drag your mouse from a point near (but not too near) the edge of the image closest to the previous image until just enough of the previous image shows through that the two images appear to be blended. If the previous image ends too abruptly you may need to do some of the same for this edge of that image as well.
When all the images have been added this way, go back to base background layer and use the rectangle tool to fill in the section under your title with the desired color (for this site I used black). Now blend the left edge of your final image to this new background the same way you blended all the previous images together.
And that should do it. You can save the project (in case you want to come back and tweak later), and export to a jpeg for use on your own site. When exporting, remember that this is for a web site and not a photo contest. You can choose a lower quality level for the jpeg, because the images are often so small already and the reduced quality will just serve to hide the blending. Don't go too far, but remember that some people are still on dial-up. My last image came out 13.8Kb. This one was 20Kb.
Thanks for reading, and I hope someone will find this useful.