If you’re managing your own website content and want to add imagery to it, you should bear in mind the following.
Resize or crop your images so that your images are of a consistent size and not too large. Large images slow your page speed down (which is penalised by Google) and reduce the user experience quality (because people have to wait longer to see the page).
When resizing or cropping your image, ensure you first change the resolution to 72dpi which is the standard resolution for websites.
Finally, save your updated image so that it is optimised for the web. This will further reduce file size for a faster page speed.
I use a tool called Photoshop which can be purchased on a monthly plan for a reasonable cost. If you intend to use a lot of imagery on your website, then it is a worthwhile investment. There is a free alternative called GIMP and likely many others. Whichever program you use, remember to use the correct file type for your image. Photo’s will be .jpg whilst graphics can be .jpg but is many cases .png is used now. More advanced users would consider .svg format in some cases.