I'm not clear what you are asking. This is simply how AE works - each image is a layer and their stacking order determines what is visible as do keyframes on parameters such as opacity. Rather than looking for ways to avoid this and work against the fundamental principles, you might want to spend some time learning this stuff...
I used Import - Multiple Files option from File menu... that's what I was looking for. One layer and all 20 images side by side as a pre-composition. Thanks!
You have two options. Import your 20 images as an image sequence, open the interpret footage panel for that image sequence, then set the frame rate to .2 fps. This will give you five second long frames when you add the footage to a comp with a standard frame rate.
The other option is to select your 20 images in the Project Panel, select Create New Comp from selection, Single Comp. Then move the CTI to 5 seconds, select all layers, press Alt/Option + ] to set an out point for all layers at 5 seconds, then select Sequence Layers from the Keyframe assistant. This will give you the option to overlap and cross face each image. Now nest this 20 layer comp in another comp to achieve your single layer goal. This is the preferred method, it's very quick, and it allows for customization in timing, framing, and effects that is very difficult with the first option.