As with any SEO task, it’s all about time. Obviously, downloading renaming and re-uploading all these files could take hours. Perhaps you could pick the most popular pages on your site (find these via Google Analytics) and focus on fixing those. Then make sure your name files are correct from this point onwards. If I had two hours to kill would I spend it renaming and uploading images, or would I create a killer new piece of content and amplify it via social networks? I’d go with the content option every time!