Are you putting the image on it's own sheet? Not that this is required, but it makes it easier to handle to avoid for example the image ending up behind the background or similar.

When you insert the image, do you properly give it two corners as the command line asks for. The two points you provide as corners determines the size of the inserted images, so if you click two points really close to each other when inserting, it will be tiny.

Also make sure the image is actually a png image. Try to open it in a local image editor or something. Lots of websites does weird things, so you may have accidentally ended up with the html code for the page saved with a .png extension instead of an image.

To comment on one other thing from your post

You can reorder entities using the ordering buttons, such as :CC2INFRONT:, :CC2FRONT:, :CC2BEHIND: and :CC2BACK:. All of these reorder entities within the same sheet. This does not fix the actual problem you were experiencing of course, but that has been discussed above, and this may help you arrange other things in the future.

Thanks Wyvern. Maps are now in the atlas.

And here we go, now part of the atlas. Thanks Shessar.

With SS6 released, I have now added this to the atlas. Thanks Jeff.