OK, this is pretty simple. If you cannot actually get those pictures into posts via WordPress 2.5’s new image management thingy, you can usually see the URL of the image you want after funding it in the Add Media > Add an image iconr.
All you do is copy the url of your chosen image from the pop up menu you see after clicking on the Add an image icon and paste it into the insert/edit image pop up which appears when you click on the little tree icon in the menu bar above your post.
Your image will then appear in your post, just the same as in the old WordPress, but now with the bonus of the image size being added automagically.
Where there’s a will, there’s a way! At least this makes 2.5 useable until the next upgrade comes along.