Can I use my own images in a post? (Essential AI SEO: AI Blog Post app)

Yes, inside the post text. In the app's editor you can add, change or remove images in the body of the post.

The featured image (the main image of the post) is made by AI from your Image description. If you don't like it, open the post and regenerate it, or remove it. To skip it, untick Generate featured image before you generate the post.