Create any image you want using any graphic or AI software you want, then using said software, make sure to resize it to max 1024x1024 pixels with no spaces (had a hard time with the no spaces one tbh...), and make sure to save it as a .png image.
Next, go to Customization, then click Custom Logo, followed by the + sign. Finally click Upload from Desktop, find your logo, and upload it.
If you're uploading a lot of logos, I strongly recommend designating one or more folders for it.