Also explained in this video.
- Images are resized on-the-fly
- Can resize images not uploaded with WordPress media uploader
- Does not create multiple images on your server (storage)
- Uses cached image(s)- some users prefer this rather than having to load same image in multiple sizes on a page
- Easier to change width & height of an image (works out-of-the-box in most themes without any setup required)
- Does require ability to write cache images on server (folder permissions)
WP Post Thumbnails
- Image sizes have to be pre-defined in theme
- A new image file is created and stored on upload to your server (if you have server storage limits, this is not ideal). 1 image can result in many images on server.
- If you ever want to change an image size, you’ll have to regenerate thumbnails for all previous posts.
- If you have existing posts, they will need to have thumbnails generated otherwise site will not have images in required sizes
- If you switch themes, it’s likely that the images will not be sized perfectly for the next theme