You need to use equal height and width according to the product documentation.
See this thread for discussion and a link to the doco here:
Modify Report an Issue and Request a Service
Here's some comments, that although for an older release SDM 12.6, and not USS, probably have validity:
12.6 Changing Banner Image Size
And you can go here to vote on a related Idea to allow customisation in this area:
Enable customization on USS
In the meantime, the best workaround is to make your image fit, and then fill in "bits left over" with an appropriate background colour.
Thanks, Kyle_R.