Images can be the low-hanging fruit of SEO. Easy to fix but often overlooked.
In general, you’ll use images in JPEG format, with PNGs reserved for logos or images with a transparent background. Vector graphics (SVGs) with ability to resize without loss of quality are good for icons and illustrations.
Don’t upload images straight from your camera. For starters, cropping with an editor such as Pixlr
will often improve the look.
I like to keep images to a 16:9 format so they don’t take up too much depth on desktop monitors, unless there’s a need to change it. It’s up to you, but the human eye likes consistency.
There’s no hard-and-fast rule about image dimensions (measured in pixels). It depends on your theme, and Wordpress automatically creates different sizes. But there’s no point uploading huge files.
What’s really critical is filesize - try to keep images under 100kb.
Some image editors have ‘save for web’ or output quality options, but dedicated compressors go further. To see optimisation in action, try one of the test images in Squoosh