I probably would not tempt doing it unless I was fully in control of the Requests that came in. I would want the image request to go to PHP to be handled. So, Request for image comes in, PHP pulls it and determines how it's going to send the content, if Googlebot, send the image, otherwise do the Header redirect with referer passed in to perserve referer content.
302 is specifically for content that is not permanently at a location, technically it's true that you're temporarily sending page requests directly to the site when someone clicks on an image, but you'll probably only do it for as long as the image is popular.
One thing I would suggest is having the requested image obviously on the page and clickable on the homepage, otherwise you'll have some pissed off users.