The simple answer is, it depends! For site owners who are looking to ensure that their sites load quickly, then it’s imperative to have a crunched-down version of the full resolution image uploaded, with a max file size of about 250kb. (Have a read here on how to resize really large images).
As a quick rule of thumb, you should aim to use the following sizes for your website images:
For large slideshows, or full-width content areas using a photo background:
Ensure the width of the image is at least 1920 pixels wide (HD) and max 250KB filesize.
For galleries:
Ensure each image’s width is approx 1200 pixels wide, and aim for 150kb filesize. This provides a nice size for the pop up version (lightbox).
For small images (that don’t need to open up into a larger image):
You can make these images quite small, depending on how big you need them on the screen; always consider users who may be using large screensizes so aim for approx 800 pixels wide – see example below – if viewed on a 2560 pixel width screen, you would need each image to be at least 850 wide if you want the row to be full width. These can be small file sizes; approx 100KB or less.

Medium Images
For a row with 2 columns, where one column is an image, you need to ensure it’s at least 1300 pixels wide; max 200KB file size – see example:

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