Generate image styles on upload or in some other memory-conscious fashion?
I don't like the "lazy" image style generation either. After I imported 100,000 photos I batch created image styles for all of them. I just looped over each image and created the 5 or 6 styles for each image.

I noticed that there isn't a big penalty for looping over images that already have all image styles generated. So you could just run a cron, say every hour, that loops over all images created or updated within the last hour (or two to be save) and create the image styles.

I used something like this to generate the image styles, just select which ones should get generated:

    foreach(image_styles() as $style) {
        if ($style['name'] = 'thumbnail' || $style['name'] = 'medium'){
            $dest = image_style_path($style['name'], $uri);
            print ' '.$style['name']. ', ';
            if (!file_exists($dest)) {
                image_style_create_derivative($style, $uri, $dest);
            }
        }
}


You'll just need to $uri variable, in my case I get it directly from the file_managed table since I ran a batch job.