It says right at the upload page: "[color=#333333][font=Verdana, Arial, Helvetica, sans-serif][size=2]The small logo is displayed on arrival and departure billboards throughout the airport as well as on other occasions within AirlineSim. The logo can have a maximum width of 120 pixels and a maximum height of 23 pixels. Accepted file formats are JPG, PNG and GIF. Due the importance and fairly small size of the logo, a pristine and readable logo is essential. Therefore the design will need to be approved by two out of three moderators before being activated."[/size][/font][/color]
[color=#333333][font=Verdana, Arial, Helvetica, sans-serif][size=2]On the settings page you also see the status. Only when it turns to accepted will your logo be displayed in these locations.[/size][/font][/color]
[font=“Verdana, Arial, Helvetica, sans-serif”][color="#333333"][size=2](I also removed a bunch of the question marks, since adding more of them doesn’t get the question answered any quicker )[/size][/color][/font]
The “main logo (300px * 100px)” appears on the general airline information page - for the logos in the departures- & arrivals-table you have to upload a small logo, which is right under the main logo-upload-window ().