Could you please explain in more detail what it is that you are trying to create?
Your attached 7Mb GIF does not contain any animation. Was it supposed to?
As Michael has said, your attached GIF is a still image and not an animation, but it is straight forward adding to an existing frame animation. When you copy a frame animation from the web, it is going to have an equal number of frames and layers like below.
To keep things organised, I like to group the existing layers. Then add the new content on new layers. By default, added content will show on all frames, but you can decide which frames to have turned on for the new content.
When done, export with Save for Web as before.
Thanka for the advice. I didn't export it with Save as Web. With the animation, I already have the images put together but then wanted to edit it further to add arrows to show what is moving, I've started doing this by adding new layers, however they don't have a background. Please see the attached.