I have a mac and drag and drop images all the time in the editor both in Safari and in Firefox. You just have to make sure that your mouse cursor (pointer) is exactly where you want the image when you release.
The main problem with the particular post you mentioned is that there is very little text to help with alignment of the images, and because there is such a variety of image sizes. You have to remember that the web is not like a page layout or drawing program where you can align things perfectly and precisely by small increments, it is still a text based thing and as such it is difficult to do what you were trying to do unless you are working in pure HTML and using web page design software such as Dreamweaver or something like that. That post is a real challenge.
I would likely have done things in a 2-column table and then alternated the images and text in the left and right columns to keep the look interesting. I would probably have also worked on getting a little less of a drastic difference in sizes on the images.