When you insert an image into a WordPress post, its display size is determined by the size setting you pick on the insert dialog:
If your server is configured to support WordPress's image resizing (most common hosts configure their servers so this isn't an issue), a resized version of the image you enter should be used so you're not forcing a visitor to download a large image and then smushing it down with HTML width and height attributes.
If you find that you've inserted a "Medium" size image but the file being download is huge still, then Regenerate Thumbnails should work. However, if you inserted the images at Full Size and your theme is making them fit the body's width, then regenerate thumbnails won't do anything for you. Unfortunately, in looking at your blog, it seems that this latter scenario is what you have.
The only way you can address this, then, is to set the "Large" size to match the width of your content area and then reenter each image. I don't know of any other way.
One other way that won't work is to change the image size in the "Advanced Image Settings":
Again, all this will do is resize the image in the browser. It won't change the file size or dimensions of the image being displayed.