You might rotate the image in SU after import as well.
I'd say the use of the image is just a reminder help in this case. Because the shape is so simple, you will end up entering the exact measurements you want as you go. If you are unaware of the original measurements, you'll be making them up to reasonable or functional round numbers. Same with the placement and size of windows and fixtures.
Use guidelines. Cannot say how many times i have drawn rectangles and they did not come out the way I wanted them to. Drawing the guidelines first eleminates many potential problems later in the process
Set groups and/or components early in the process rather than going back later in the practice fees and trying to make portions and trying to set them.
Again, set tags early in the process.
In particular, these tips are great for beginners. That are also good reminders for experienced users. 😉