For visitors of your Web site it’s important if the pictures they see there are as light as possible, so they’ll load as quickly as possible. If a picture is big that’s one thing (w x h in px), but it shouldn’t be too heavy (weight in kb), especially not for those who are using their cell phones. Thankfully, there are quite a few tools to improve the compression of a picture and one of them is this online one with a friendly panda:
If you don’t want to make your picture smaller but you do want to make it lighter,
you can use tinyjpg.com for JPEG/JPG
and tinypng.com for PNG
With some pictures the improvement will be bigger than with other but I nearly always use this service.