Sounds like the off-center rendering of Serato DJ screenshots on your MBP with Snow Leopard is likely a browser display issue rather than a problem with the images themselves—sometimes Chrome can misalign images depending on scaling or page formatting, so trying a different browser or adjusting zoom settings might fix it. Meanwhile, for reference on how images and media can be showcased correctly in a gallery-style layout, you can check out examples like this
baddiehub.news, which handles media rendering cleanly and could give you an idea of proper alignment techniques.