I would normally put this in the Help Center, but there have been so many questions about it, I thought it would be seen here by more members who might be interested.
Only members can edit their own Blogs. Hover your mouse just after the title of the post. A pencil should appear. Click on that and then you get the same tools you use when originally posting. Unfortunately, Attached images cannot be resized within the post. You have to Detach it and resize on your own computer. Then you can reattach it. Inserted pics [using the picture frame icon] can be resized once they are uploaded but not yet submitted. In How to Upload (Post) Photos scroll down to my post. This explains what you can do with Inserted pictures. I much prefer Inserted pics to Attached pics because you can put titles on the Inserted pics and intersperse the pics throughout the post, such as when you want to write a tutorial. Attached pictures always appear in a group at the end of your post.