Media

Image sizes enables you to specify at exactly what sizes your images will be published.

This is important because browsers work more effectively if they know exactly how large an image is supposed to be. It enables them to allocate that exact amount of real estate on the visitors screen into which the image will load – even if the image itself is a high resolution and slow to download.

A Thumbnail is a small representation of the image. It allows the visitor to get an impression of the image without having to slow down the page loading by downloading the whole image file.

When publishing images in your posts and pages you will be asked what size you would like the image to be; thumbnail, medium, large or actual size. WordPress will then process the file into the dimensions you specify here.

Bearing in mind the size of the average users screen then 1024 is probably about as wide as is useful to use for all of the image to appear on the screen.

Don't forget to SAVE CHANGES.