If you want your website to load quickly for your visitors (and believe me, you do), it’s important that the images you use are sized correctly for the web. If you take a photo of your latest sculpture in stunning hi-definition and then throw that directly onto your site, your visitors are going to watch that photo load on their screens bit by bit. Bar by bar. Like it’s 1996. More likely, they’ll just leave.
Image size is increasingly important as more and more users are viewing your website on their smartphone. The bigger your images, the more of your visitors’ precious data you are demanding.