Click "Render" above to enter the rendering interface. Go to "Environment-My" and click "Upload" in the left list, then type in the name for the uploaded environment in the pop-up window. Then just click the "Click to Upload" button to select the image and hit "OK" to finish uploading. Once the upload is complete, you can check the status of your uploaded environment in the left list "Environment-My". And you can apply it to the exterior view of your design for rendering when the process is finished.
*Note: when the uploaded image is in 2:1 aspect ratio, it will be processed as a spherical map by default.