You can set logo in Main menu > Settings > Logo. Logo should be enabled in printout settings too: Main menu > Settings > Printout > [Select printout] > Header logo / Footer logo. The logo size should be not too large if you want to keep your database and your PDF files small (e.g. 250x250 or 1024x180, ...).
Supported formats are: PNG, JPG, JPEG. If your logo has sharp lines then PNG format is recommended. If your logo has gradients and smooth areas then JPG is recommended.