I upgraded to WP 3.0 via the automatic update. I have a single user blog. Before the upgrade I was able to easily insert pictures into my posts (after uploading them into the library of course).
Since the upgrade I have been unable to insert any pictures into the post. I upgraded WP and then I upgraded my them (Genesis). I have downgraded the Theme and still have the problem.
I have tried deactivating all of the plugins (I believe I have 4 active) and I still am unable to insert the picture into the post.
I can utilize the featured picture option. Which means when you are on my blog page http://www.nicolerosen.net/blog; the pictures show up. But, when you click on a specific post within the blog and WP switched to the page for that blog post - the image disappears.
I am not a code junkie....I see other people are having similiar problems, but most of the have to do with upgrading from WordPress MU.