Using the clinic's logo is a great way to brand the app.
FYI: The logo needs to be less than 20K in size to be used as the logo.
Click All Tabs (+ sign)
Click Documents Tab
Click the New button
Enter "PracticeLogo" for the Document Name and Document Unique Name
Check the Externally Available Image checkbox
Set folder to Public Images
Choose your logo file to upload
Now, goto Setup.
Search "Apps" or goto Build->Create->Apps
Click Edit on the Practice Management row.
Under "Choose the Image Source for Custom App Logo", click "Insert an Image".
In the pop up window, select "File Location: Public Images" and click Go.
Select the image named "PracticeLogo".