google ranking is not connected to sitemaps. sitemaps simply help search engines with discovering new urls. on page factors (like keywords) and off page factors (like backlinks) impact your rankings.
most sites do not even need sitemaps. sitemaps do not hurt so people liek to use them just to be sure they are doing everything they can to help their site.
when using sitemaps i would advise you to keep things simple. google is not the only search engine. the simpler everything is the better your chances are with all search engines. that is why i would personally build out a separate image sitemap.
if you always underestimate the capabilities of the search engine crawlers you will not be disappointed (unless you are trying to hide something :).