BD sent over her instructions for working with PS or PS Elements, but I've worked out a little procedure for preparing these banner photos using the GIMP. These are the steps:

1. Choose your image.
2. Crop and scale the image so you have something which is 508 pixels wide, and about 196 pixels high, including the bits that are going to stick out.
3. Make sure you have an alpha channel in your picture — Layer / Transparency / Add Alpha Channel./ If it's greyed out, you already have one.
4. Select a rectangle which will comprise the rounded bounding box, out of which the sticky-out bits will stick.
5. Select / Rounded Rectangle..., then set it to 15% and hit Apply. Your box should become nicely rounded at the corners.
6. Select / Invert, to select the rest of the image, then hit Control + X to cut, and then File / Create / From Clipboard. This will move the cut bit of your image to the clipboard, where you can work with it in a minute.
7. On the original image, choose Select / Invert again, then Edit / Stroke Selection. Set the width to 1 pixel, and press OK. You should now have a rounded rectangle with a border around it.
8. Switch to the other image, and do whatever you need to do to isolate the sticky-out bits. One approach is to use the lasso tool to outline the bits; you could also carefully erase the background using an eraser.
9. When only the sticky-out bits remain, and the rest of the background is transparent, draw a selection rectangle around them and hit Control + C to copy to the clipboard.
10. Switch to the other image and press Control + V to paste the protrusions back into the image. Position them carefully over the image so that they cover the border line and line up with the main image correctly.
11. Layer / Anchor Layer, to fix the pasted bits in place.
12. Save as XCF, in case you need to edit later.
13. Export as JPG (no transparency), or preferably PNG, which will preserve your transparency.