Before uploading a custom logo to your stream, ensure you've subscribed to the Premium plan of Yostream. With the Free pricing plan, you'll get only default logos and can't upload your own. If you try uploading with the Free plan, the following message will appear:
To proceed, check out the different Pricing Plans of Yostream and select whichever fits your streaming goals. After upgrading the plan, you can upload your custom logos.
After entering the Yostream Studio, you'll see the following screen.
Click the Brand icon in the bottom-right corner.
Click Graphics.
Click your desired logo so that it can be displayed on your screen.
Your logo has been successfully uploaded to your stream.