Digital Light & Color

There are other ways to do that, but I tried joining two images using Add Border. The image on the right is 2496 pixels wide. I believe I made the height for both of them equal at 1636 px. What am I doing wrong?

Note that your border image is ...

an image that is tiled to fill the background behind the image.

Oh, so you're saying that the 'background' image starts at the left edge of the base image rather than the right edge of the base image when added as the right border?

That's right -- this feature is designed more for creating a textured border as defined by a seamless texture tile.

I'm not sure what effect you are trying to create, but you might want to be using a combination of Add Border and Composite or possibly Layout.

I think I understand it better now. Is this true: if I add a Border, regardless of on which side(s), and choose Border Color to be another image, this border image registers its lower left corner at the same pixel as the lower left corner of the base image and replicates or crops itself as required to fill the area of the whole output image with borders?

I believe the tiled border image aligns with the lower left corner of the output image.