If someone is offering you SEO advice that doesn't feel quite right - if what they're promising sounds too good to be true, it probably is. Remember, no one can guarantee any organic results. Diligence, work, and data-driven strategy are the only ways to play the long-game of SEO and improve your page rank.
Striking the balance between clean, crisp, high-resolution images and website load speed can be a real struggle. On one hand, large high-res images look better; but the downside of such quality is significantly slower page load speeds, which will make your users and Google take notice - and can have an impact on both user conversion and your SEO.