An image in a blog post is more than decoration. The original article identified three jobs: invite someone to read, help discovery and support the text. That framework still helps, provided you choose an image because it adds something real.
Attention without a false promise
On an article index, a suitable image can help readers recognise a story. A generic stock photo unrelated to the topic does less. Choose a visual that reflects the question or example in the article. The first impression should fit what follows.
Let the image explain something
A diagram can clarify a process, a photo can show a difference and a chart can make change visible. Explain in the text what readers should notice; do not assume everyone draws the same conclusion. Add meaningful alternative text when the image carries information so people who cannot see it still receive the important point.
Discoverability is not guaranteed. A clear file name, surrounding context and appropriate alternative text help search engines understand the image, but they cannot rescue a weak article. Write for readers first and use SEO as an extra quality check.
Check usage rights
Use your own images or material for which you have permission or a suitable licence. Read the terms: attribution may be required and commercial use may be restricted. Appearing in image search does not give you a right to reuse a picture.
A good image attracts attention, carries meaning and fits the page. If you have nothing that does one of those jobs, a clearly written article without a photo is better than random decoration.