When import image into VSE, it is true that the original aspect will be changed to match the scene resolution. To fix this, you can simply add another **Transform** strip to first Transform (NOT the picture strip itself).

So, basically one for Uniform scale, and another one for aspect ratio correction.

![enter image description here][1]

![enter image description here][2]

![enter image description here][3]


**P.S.**: When setting the second Transform scale, be sure to set the longest side to 1.000, which is **X** in this case.  


  [1]: https://i.sstatic.net/1cJ4j.png
  [2]: https://i.sstatic.net/SYTPF.png
  [3]: https://i.sstatic.net/sNTpN.gif