I had the same problem. The moment the page starts to 'freeze' and the message 'waiting for local host' appears, you can synchronize your modeler by pressing 1. Next images will be uploaded as they supposed to.. This is off course a temporary solution. I handed in the problem and it will be solved in the next update.
I get a similar issue under IE7 (which doesn't occur with Firefox 3).
I have a form with various fields and a Image Uploader.Clicking OK on the form with no image selected works fine.
However, selecting a image works causes the image to be uploaded and linked to the object correctly but the application immediately freezes on pressing the OK button and needs the user to refresh the browser and log in again before they can continue.
There is nothing logged in the Mendix Modeller or the log file to suggest what the problem is.
I had the same problem last week with my file management. I think this is because you use the same form both creating new objects and editing existing ones.
The Upload File widget seems to always need an input, even if the object already has a file attached. I fixed it by splitting New and Edit in to two forms, New having a file upload widget and edit shows the filename of the attached object instead of the widget. This way you can still change any other attributes on the object, but you can't replace the object already attached. You could add a third button Change File that calls the New form with the upload widget on an existing object.
I have tried to apply this as a solution to the issue in the original question, however it does not solve the problem. The freeze occurs after adding two objects consecutivley both with images uploaded against them, creating an edit form with no ability to upload image dosn't fix the issue as the edit form is never opened in the steps followed to reproduce the bug.