To optimize for Web will have several components. First, the physical dimensions that the image will be displayed at. In your case 600 X 450 placed as they are on you page does not display the entire image without scrolling on my 1024 monitor. A slightly different placement or slightly smaller image dimensions would solve that and could result in smaller file size.
Second, who is your audiance and what expectations of a high quality image are they expecting? An artist or photographer will "optimize" their images to show off their skill, and fine detail work, using much large image files than would, say a construction company posting images of the latest phase of building. Other applications would "optimize" to speed download time. But remember, take that in the context of what other page content is be downloaded at the same time. A 250kb image downloaded with 1500kb of other content....well maybe everything needs to be reduced a little. A 250 kb image and 50 kb of other content...I wouldn't bother.
Your images range from 86 to 276 kb, loading one at a time. It's up to you but I would think that all of those are within reason as far as file size goes. If you do decide to "optimize" by slightly reducing file size, just do it for the couple over 250 or so.