Using the wrong size images
Images which are too big for the container they’re in or are heavy on the pixels will be penalised by Google, affect your SEO ranking and add vital loading time to your website.
Images which are too small will pixelate and blur, losing the detail and making your website look amateurish.
Neither of these are great. Do the work to your images before uploading them to your website - make sure they’re the right size for the maximum input of your site (on WordPress this tends to be 2560px), run it through a compression tool or save it as a web-specific file and finally, select the size you want to use it as, when adding it to the page. In the case of lo-res images, if you can’t find something of a high enough resolution then change the image, don’t put a low resolution image in and hope for the best.