It really depends on the expected conection speed of the viewer of the web site. For instance, a web site that is for gamers, you can pretty much assume they have a fast connection, so you can use larger images. If you are doing a website for senior citizens to get free digital cable boxes, then I would consider the possibility that they may have a dial up connection and you might want to use smaller images.
With dial up connections they used to say an entire web page should be less than 50K... then broadband connections became more popular and it was more common to see images 200K in size as long as there are only one or two on the page. There is no right answer. You want images that load fast and look good and there is a delicate balance between the two.