Some guidance about when to post large images
If a thread's purpose is to display images (example: a "show and tell" thread in the Digital Photography forum), using IMG tags to display large images is perfectly suitable.
Otherwise, try to limit the sizes of images you include (with IMG tags) in posts, because it slows people's page displays and many viewers don't have large monitors anyway. When the image isn't critical to the discussion, you can link to the image rather than display it directly, so people can choose whether to display it.
When you attach an image, a thumbnail is shown for the benefit of thread viewers. They can see a small version and click if they are interested in seeing the full image. They don't have to pay (in time and screen real estate) for it ahead of time. If you use the trick of putting IMG tags around the address of an attachment, you are defeating the software that is there for a reason.
My pet peeve: People who quote posts with large images and leave the image in the quote.