Waggin

Most images I see in posts show up as a reasonable size, but you click once, and it opens by itself on a black screen, and another click expands it to full size.
If I try to post a full size image, it fills the whole forum page, and is not reduced to the usual thumbnail I have to reduce their size before posting, which then prevents them from being enlarged to full size.

Here's a big one,. How do I get it to show up much smaller in the forum post, but still be expandable with a couple clicks?