For anyone else looking for a real answer to this question, here are the directions (for Commerce Kickstart Theme 7.x-2.31 users):
1. On the top menu, go to Site settings > Visual & Layout > Appearance.
2. Select "Settings" under Commerce Kickstart Theme 7.x-2.31.
3. Select from the four settings "Global settings", "Bartik", "Commerce Kickstart Admin Theme", or "Commerce Kickstart Theme" to change the logo file selection.
Just for reference, the default logo image size is 192x56px. If you upload a big image, it will not automatically resize.