I've just had a look at your site. You don't appear to be using NextGEN Gallery for your post images, but rather the image uploader/gallery built into WordPress. Also, you seem to have been experimenting with different image sizes and the way they're inserted, so I'll just focus on your latest post here to give you an idea...
If you want your images to display side-by-side, you'll need to select the default Alignment: "Left" option on the Add Media screen when adding them in your post editor. This will float your images to the left alongside each other until there's no more room in the post content area. You've currently got them set to "center", which breaks each image onto a new line.
Additionally, you'll want smaller image sizes as your current images won't fit two across (they're 300px wide). If you select Size: "Thumbnail" on the Insert Media screen instead of "medium" as you have them now, you should be able to fit three images across.
Hope this helps!