When writing a post in WordPress, and you press Upload/Insert buttons to insert images, video, music or other media you can run into a problem where a popup displaying “Are you sure you want to do this?” is showing instead of the normal insert screen. The problem is described here. As mentioned in the posts, it is caused by some plugin. This can be hard to guess as the plugin in question might have nothing to do with writing and posting articles.
I ran into this problem using the latest WordPress 3.0.1. The plugin that was causing trouble for me was WP-ContactForm. By disabling it, it was possible to insert images in the posts again.
If you experience this problem, try disabling all plugins. If inserting images now works, try enabling the plugins one by one to figure out which one is causing the trouble.