If your images are all the same size this process should be done very quickly and you do not need to export any precomps. Make your black still 1 sec long, select it and double click on the normal dissolve transition to place it on both sides of the black frame. Now create a black still the same size as your images, place that still between your two images and copy and paste the resize and positioning from one of the images to your black still.

Place them both in a secondary and size and position them over your BG. Lets say you have two images in the same size. (the reason the 'fade to colour' or using a not modified 'colour solid' affecting the whole BG is that they are actually generated in the size of the project and because they are auto generated that size will always cover the whole frame, no matter what size your project is) Watch this video to see how you can quickly apply fades to a video clip using Final Cut Pro X's Video Animation Editor.

The thing you have to do is to reframe (or better create as a still) the 'black' solid between your images to the same size as the image. Fades to and from black are simple in Final Cut Pro X. You can actually do it all in the same project.

Download fade to black final cut pro