For proper instructions please read this by BigCommerce, https://support.bigcommerce.com/s/article/The-Bigcommerce-Themes-Marketplace?language=en_US

Uploading Images

1. Go to Storefront › Image Manager, then click Upload Images.

2. In the popup that appears, click Choose File (or Browse) and select an image from your computer.

3. Repeat Step 2 for each additional image you want to upload. When you are finished, click Upload.

4. After processing, new images will appear under Image Manager.