Resolution

The main rule here is that you should estimate how much space certain images take on your page. For example, if you have an email that is 800 pixels wide and your image takes half of the email, then there is no point in having images that are more than 800 pixels wide(Example). Moreover, you might even consider having image that is 400 pixels wide, depending on the situation. So, let’s divide images into 2 groups:

Background images: here you can have images with resolution up to 2,000 pixels wide. This type of image usually takes all screen space from edge to edge. Since most of the screens are 1920x1080 then we should be close to that number(1800-2000px).

Content images: regular images that you add to your content. Usually, those are 2 times smaller than background images. General rule is to not have a content image wider than a page container. Example.

Size

According to companies that provide website optimization tools, average website page contains images worth of 1MB. Total page size on average is 2.2 MB.

We encourage you to not have images that are more than 1MB in size. For the best performance, all your images on a page should be less than 1MB in size.

Compression

Images contain a lot of information that can be used to adjust their contrast, saturation, and brightness. There is no point in having images with that information on a page. Here is where compression comes in. Upload your images on https://tinypng.com/ and get rid of excesses.