I've tried testing uploads from both a Mac and a Windows machine, and under multiple users. The uploads is working. One thing I noticed was under Windows Internet Explorer, with a certain security setting I'd have to use the older manage attachments popup which doesn't give an upload status indicator, meaning you don't see anything happening while the file is uploading. With a larger picture it may take a while to upload the picture during which time it might look like it's frozen as the screen is just sitting there loading. However if your browser is modern within the last few years, and you are running JavaScript and don't have high security settings which can also interfere with the popup window, then you shouldn't have to use that old basic uploader.
Another possible issue is that there may be an advertisement running on the page causing a script conflict that is hanging up your browser. I haven't come across one in testing, but the thing with advertisements is that they can be targeted geographically and to specific browsers / OSes, etc.. So, some users may be experiencing an issue caused by a bad advertisement which does happen on occasion, but the only way we can address that is to know specifically which ads were running on the page at the time. We don't manage the ads, we just run a Google AdSense tag that randomly serves up ads from it's own roster. In this case, we really need to see a screen grab of the entire page when the freeze happens.
