When I train clients to use WordPress, I stress the importance of resizing and compressing images. Resizing means changing the dimensions of the image (say, from 4272 × 2848 px down to 1024 × 683 px. Compressing means removing data from an image, such as removing metadata, reducing color depth, and reducing fidelity.
So images and pages load quickly on slow connections (especially mobile devices)
A smaller total file size for the site reduces the amount of storage space taken on the host and backup destination
A smaller total file size for the site helps backups run quickly and smoothly.
Rules of Thumb
In general, I recommend keeping images below 250 KB whenever possible. You can use the image editor of your choice to resize and compress images before uploading them to the site. I generally resize images to a max width (for landscape) or height (for portrait) of 2048 px, and save JPEGs at 80% quality (compress them) to get them down to around 250 KB. For specific steps, see the documentation for your image editor.
Let Imsanity Resize and Compress
The easiest and most effective way I've found to automatically resize and compress images in WordPress is the plugin Imsanity. Here's how to use it:
Settings > Imsanity. Leave the max width and height for Images uploaded within a Page/Post and Images uploaded directly to the Media Library as default (2048 × 2048), or change them to match your theme (for example, width of 1140).
Set Images uploaded elsewhere (Theme headers, backgrounds, logos, etc) as default (2048 × 2048), or change them to match your theme. Or, set to 0 to disable resizing in these cases.
Set JPG image quality to 80. Or, set it above 80, if you need images to look crisp on your site (such as a photography site or a retail site).
Click Save Changes.
Now any images uploaded to the site will be automatically resized and compressed.
Bulk Resize Images
Imsanity can also resize and compress images that are already in your Media Library.
Back up all media (or entire site). We use ManageWP for backups, but you can also use BackupBuddy.
Settings > Imsanity. Scroll down to Bulk Resize Images and click Search Images.
Select and deselect images to resize and compress.