Click on Participants in the left sidebar and open the Teams tab. Now make sure you have the Logos column enabled from the Team Information panel. Then click on the edit pencil next to a team to upload a logo.
There's currently no way to upload all logos in bulk or to reuse logos from other tournaments. You have to upload logos to each team individually.
We recommend uploading transparent .png images to get the best results. The logos will be compressed automatically to reduce the file size.