Lock aspect ratio means Tableau won't change the aspect ratio of the image when it is displayed on the viz. But the mapping from image coordinates to viz coordinates does not need to have the same ratio in both dimensions.
Usually I recommend using the actual pixel size of the image as the coordinate system for the image. Set left to 0 and right to the image width in pixels, set bottom to 0 and top to the image height in pixels.
But if you've already set up the data in some other coordinate system, then that is how you need to set top/bottom/left/right.
Can you post a TWBX or a more detailed description of why you having problems setting top/bottom/left/right?
Thanks for the response, that makes sense. I can't post the TWBX because of the nature of the project, but everything is set up in a latitude/longitude coordinate system. I think I was just getting frustrated because I couldn't seem to get the coordinate system correctly aligned with the plotted points. However, I appreciate the recommendations and explanation on how the image coordinates and aspect ratio can work hand-in-hand.