Unfortunately, setting the memory limit in PHP is only an advisory. The server may still limit the memory allocated to the script to some smaller amount. So you should try resizing your image to WEB sized dimensions and see if that works. If not, there is probably some compatibility problem with the image and the GD library zenphoto uses.
Last night's nightly build has added some more debugging information to the image processor. You can install that build and change the `IMAGE_DEBUG` define (functions-basic.php) to `true`. This will create a log in the zp-core folder named debug_log.txt containing information about the processing of the image. Browse to the image which is giving the problem,delete the debug_log.txt file, then refresh the image. This process gives a log of just the processing for that one image.