By default, your new SharePoint site comes with a default SharePoint image as the logo. To start branding your site, you might want to add your own logo. This guide will help you accomplish that.
On the right hand side of your main site, just to the right of your name, you will see a little jog wheel, click that, and in the dropdown, go to Site Settings.
Under the Logo and Description part, depending on where your image is, you need to click on From Computer or From SharePoint. If you have already uploaded your logos to your SharePoint site. Maybe in a picture library or in Site Assest library, then you would choose "From SharePoint" but if the image is still on your computer or local network, then click on From Computer. On the next screen, locate your image, and then click OK. Your Logo will now be active across all your sites. You can only have 1 logo per site collection.