Blog assets are the logo and avatar you've chosen for your blog. This is not the same image that appears in your byline when you publish a post or reply to a comment, but the images that appear at the top of your blog home page. By default, the avatar will display your personal avatar, and your blog will not have a logo.
To update your blog assets, click the User Menu while on any page of a blog which you own, and select the Manage option:
On the manage blog screen, select the Settings tab:
Then upload your files:
Uploading at the recommended dimensions will assure that your assets will display at the highest quality across all screens and devices.
Pro-tip: Upload your image in PNG format for higher image quality.