Umm... if it's a zip of images, it really shouldn't be that large (maybe a few megs). Your solution isn't going to be one of AJAX or Flash as that's just the GUI to the upload. Your issue rests with PHP itself: What is the maximum your PHP will allow you to upload? If it's only 8MB, you could have an issue for someone who wants to upload 1000 photos in one zip. But really you should limit the upload size so that your server doesn't get too bogged down.
Now, after they're uploaded, everything is PHP and Server-side. So AJAX won't help, nor will Flash. The only thing they could do is maybe show an indicator which you can do with PHP by display a page to the browser, then processing the PHP and then redirecting to a new page.
Anyway, the issue is not what GUI you'll use, but the memory limit your server has and how well your server will handle the extra load. Think about 10 users uploading one zip file at about the same time with 50 images each. That's 50 zip files to decompress, 500 images to resize, and 1000 images to move, plus 1000 DB updates. That's a lot of work for some servers. So you may want to take that into account.
