The images that you upload through your Dashboard can literally make or break your website. If you are trying to upload very large image file sizes, it's not uncommon to experience slow page loading issues and other types of serious website and server failures. For this reason, it's very important that you stick to a reasonable image quality and file size limit for the images that you want to upload. Best practice is to make sure that your image is set to 72 dpi, and put through some kind of an image saver program. Remember, the more images on the page, the heavier the page will be and the longer the loading time. Realistically, an image can be set to a file size as small as (or smaller) than 50 KB, depending on the type and size of the image, and still look great on the Web. On the upper end, make sure that your images never exceed 500 KB in file size.
Fortunately, there are numerous, simple ways to conveniently resize your images. Here are a few options mentioned and you can research more depending on whether you have a Mac or Windows computer.