Unoptimized images can really hurt your site speed rankings on google. Lets say you have a small 150x150 header logo, but your developer saved it as 1000x1000 and it's just being shrunk to 150 in the browser. That's a whole lot of wasted Kb the browser has to download. In a nutshell, just size them properly and send the actual images through a lossless compressor
If you use icons anywhere they should be sprited so it's a single image downloaded.
Your logo should ideally be part of that sprite or an SVG. SVG is optimal because it's quality infinitely scales.
Tools like Google's [Page Speed Insights] (https://developers.google.com/speed/pagespeed/insights/) should be able to tell you when you have bad images on your page.