If you want to you can chose another file as default logo. The file has to be uploaded before (admin/file/list) and has to be one of these:
,bmp, ,gif, .ico, .jpeg, .jpg, .png (these are just the default ones, you can change them in the configuration file: instance/production.py)
The displayed logo has a max-height of 120 px. If you selected a higher one the browser will try to scale it.