If you are uploading your images to your website, it is important to consider the size in both dimensions and file storage size – this is called optimizing your images for the net.
It is important to have your images at the recommended pixel size. If your image is too small, WordPress may stretch your image to fit to its given position on the page and this will result in a blurry image. If your image is too big, WordPress will use their resizing algorithm to reduce the size of the image to fit into the page which may display an image which isn’t as sharp as you would like, and may also serve up the big full-sized image in the background when the web page loads, leading to a slow loading time and a poor experience for the viewer – perhaps making them click away from your site in frustration.
How to resize for your website.
Your website theme will have a recommended size for the dimension of the header/featured images which you will be able to find on their Info and Set Up pages. Go to themes from the left-hand menu, at the top of the page you will see links to the Info and Set Up pages of the theme you are using.
If you are uploading an image to WordPress for insertion into the content area of a page and your theme’s info doesn’t state a recommended pixel size, I would resize your image to a width of 1600px or there abouts.
To keep the file size to a minimum, when resizing your images always use a resolution of 72ppi (pixels per inch). If your image is straight from a camera it may have a resolution of 180ppi or 240ppi which is required to produce a quality print, but for the net 72ppi is sufficient and any higher resolution will increase the file size unnecessarily leading to your image loading slower on the page and using more of your WordPress storage space.
If you don’t have an editing image programme or app, use an online editor such as that provided by Befunky. With this free online resizer you can specify the number of pixels and the resolution will be reduced to 72ppi when you save the file as jpg.