This is possible - I assume you're using the page template "blog post" or adding a blog post directly to that section in Confluence?
The option should be along the top editing options similar to a normal Confluence page - is there no ability to drag an image onto the blog / add an attachment to the blog post?
If easier, please share a screenshot of your screen so we can review further.
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event