Hi Ally,
Thanks for the picture. I see you still have your background in 3D and that's the problem.
Go to the Background layer and click the 2D/3D icon to switch the layer from 3D to 2D (see attached pic). If you get a warning, just click OK. Now your background will be 2D and the camera will not have any effect on it anymore.
To use the Camera dolly behavior your photos must all have a different place in Z-space (e.g. pic 1 = zero, pic 2 = -500, pic 3 = -1000 etc...). Then you apply the Behavior to your camera.
If you send me a PM with your e-mail address I will send you a complete Motion project folder with animated photos on your background, so you can see how to do this.
Best wishes,
Ronny