What does the ‘Properly size images’ warning mean?

This warning means you are serving images far larger than they are ever displayed - the browser downloads a giant file and then shrinks it on screen, wasting the visitor’s data and your load time:

• The classic case - a 4000-pixel-wide photo straight off a phone or camera shown in a 600-pixel-wide slot.
• The visitor pays twice - they download the full file, then their device does the work of resizing it.
• The fix is to resize before upload - save the image at roughly the size it displays, and serve smaller versions to phones.

This is the most common and most fixable speed problem on small-business sites, and you usually do not need a developer for it. Resize and compress your images to the size they actually display - it is often the single biggest speed win available.