When providing guidance to clients who are selecting images to use on their sites, there are several factors to consider. WordPress will create thumbnails at the appropriate cropped sizes (as defined by the theme developers) where they're called for in the design but in general:
File Size: 1MB or below (you can upload images up to 2MB in size but policy-wise 1MB is a better goal)
Resolution: No more than 72dpi (that's the maximum resolution images can be shown at on the web in general)
Dimensions: The largest dimension (either width or height) WordPress will accept is 2560px, so anything over that will get automatically cropped on upload down to that size.
Once the final designs are complete a list of every thumbnail size the site would use will be available, but in general you want to load the largest images as possible in terms of dimensions to match the biggest thumbnail size. As long as the first three points above are considered, though, they'll be fine for selecting and uploading images.
It's generally better to have WP scale the images down (taking a 1024x968 image and putting into a 500x400 container), rather than asking it to scale them up (taking a 500x400 image and trying to fill a 1025x980 container).
Formatting & Uploading Images
Orientation: Landscape orientation ie. 3:2 is generally preferred in order to fit our standard thumbnail sizes.. If portrait images are used, please make sure they will look okay in the landscape thumbnail preview (e.g., the person’s eyes aren’t getting cut off).
Dimensions: To maintain image quality, upload images that will be about double the dimensions of how they should appear on the site. For example, if you have a card that contains a 400x200px image on the site, upload a 800x400px image.
File size: Although we want to maintain image quality on our site, we also need to be mindful of file sizes. Please avoid uploading large, unoptimized images. 1MB or below.
Large files bulk up our web storage and can lead to slower load times. More tips.
File naming: To make images easily searchable in the Media Library, use brief descriptive file names, ideally 3 to 5 lowercase words. Replace blank spaces with hyphens (-) and avoid special characters like punctuation. A year or date might be included at the end, as appropriate. More tips.
Examples:
star-route-farms-annabelle-lenderink-2022.jpg
crumble-whisk-strawberry-cheesecake.jpg
rainbow-chard.jpg
Foodwise-teens-luz-garcia-market.jpg
Alt text: For accessibility and SEO, it is a best practice to add a brief description in the “alt text” field to your images when uploading to WordPress. Keep alt text as brief and descriptive as possible (e.g., “Farmer Rudy Jimenez of Green Thumb Farms,” etc.) More tips. Please remember to add alternative text for images. It is an important part of keeping our website accessible to all.
Captions: Captions are not required, but can be used when necessary for identification or photo crediting. For captions, use the caption line directly under the photo.
Cleanup and maintenance: In general, please work to keep our WordPress Media Library clean and tidy, and avoid file duplication. Before uploading a photo, please search the Media Library for keywords first to make sure the photo you’re uploading hasn’t been previously uploaded. If you accidentally upload more than one copy of the same photo, please delete any duplicates that are not in use on the site.
Cropping Images: There is a cropping feature in the WP Media Library. You can create a cropping and set it to impact all images or only to the banner image or thumbnail. Editing images using an image editor outside of Wordpress does give you more control.