Here are a few tips for you about using/uploading images to your WordPress site.
First, web browsers do not render 300 dpi, so for all you photo fanatics out there, stop uploading uncompressed 300 dpi images to your media library. A browser renders only 72 dpi regardless of the resolution of your images. Yep, I know it reduces image quality, but only to you! Remember, your perception is your reality. The person viewing your image online doesn’t care whether it’s 300 or 72. They just want to see the image. Sure, that doesn’t help photographers or art galleries much, so you’ll just need to go old school and FedEx your printed books instead, if you want your intended audience to see the full resolution image.
Second, reducing the dpi also reduces the file size. If you have 300 dots per in, then reducing it to 72 dpi is only going to help your website visitors download your images faster, especially over mobile phones. Remember that we all have finite mobile bandwidth, except those who pay for unlimited. That means you are burning up your website visitors mobile bandwidth allotment (if not on wifi) by not compressing your images. If I were looking at your photos, then waiting for them to download on my phone and it’s not fast, I would leave your site and go somewhere else. No one wants to wait for your huge images to download on their phone.
Third, you can certainly reduce the dpi and that will compress an image, but remember the physical landscape of the image itself should only be sized to what you need to display on the web. That’s probably around 1800 pixels. I’ve seen some clients upload photos that are over 3000 pixles wide! Resizing your images BEFORE you upload to your media library is important. Fortunately, WordPress now provides a resizing tool inside the Edit feature of your WordPress Media Library, so you can resize photos down. Note: Never resize photos up or you will literally be stretching the photo like a rubber band. That will stretch the pixels in the image and your image will look like crap. You always downsize. Never upsize!
Fourth, you can use a tool like WP Smush, which is a freemium plugin, to compress your images to the best possible size and resolution. When you install WP Smush, you can compresses up to 50 images at a time with the free version. You’ll have to keep clicking if you have more images to compress. It will also not compress images over 1MB, so to process all images and images over 1MB, you’ll have to upgrade to the paid version.
Fifth (and maybe it should have been first), under Settings > Media, you can set the sizes for Large, Small, and Thumbnail images when you upload them. WordPress will retain the original file, but also copy and store resized versions to select for posts/pages. While this does not “compress” the image, it does help with managing the sizes you want to set for your site.
Sixth, remember that PNG is for transparency. You might use PNG for a logo, a small icon, or some other small graphic that may have a special use case, like a drop shadow. Don’t use PNG for large photographic images. It adds data to the image and therefore increases the file size. If you have a photo, always used JPG. There’s no reason to use PNG for any photo.
Lastly, use a CDN (content delivery network) to speed up the delivery of images on your website. With plugins like W3 Total Cache or Super Cache, you can send your website’s image to servers around the globe for storage and retrieval at the “edge” of major cities, so that they are served quickly to your intended audience. If you’re using JetPack by Automattic, you can turn on Photon, which is powered by Automattic. Photo is the CDN employed by WordPress.com, so you can leverage Automattic resources to store your photos on servers around the world. The caveat here is that it most likely only store and serve images uploaded to your media library. If you have images in your theme’s folder, they may be ignored by whatever solution you use. You want to choose a theme that doesn’t store images in the theme folder, or they’ll just be forgotten. The’ll then show up on a Google Page Speed Insights report telling you they need to be compressed, but WP Smush only compresses images in your media library and not extraneous images in theme folders.
If you need additional custom image sizes, you can use a plugin like Simple Image Sizes to create additional settings for you to select when publishing posts/pages: https://wordpress.org/plugins/simple-image-sizes/
Here’s the link to download WP Smush:
https://wordpress.org/plugins/wp-smushit/
Note: The Featured Image above is set to 624 KB and 1800 × 916. That means, it will size for most large screens and will automatically resize in mobile responsive for smaller screens. Compression will help the image load quickly on mobile devices.