If you are getting an upload success message for images in the image manager or photo galleries but the images are in fact not being uploaded, this may be an issue with your hosting account.
Some hosting companies will use programs to filter sessions based on user agents or other information. This breaks image uploading as the ability to select multiple files to upload at once comes from a flash object, which does not pass along your browser's user-agent.
Another common problem is hosting companies will share a single sessions directory among multiple accounts. Specifying a session directory for you own hosting account may fix the image upload issue.
To resolve either issue you'll need to contact your web host and ask how you might specify your own session directory and also ask the question: "Do you have any user-agent (or other) filtering processes that would likely prevent a session being restored in PHP?"
