For anyone wanting to post pictures, most of us know they have to be "hosted". Here's a host site I recently switched to that requires no joining, no sitn in, no security codes and you can drag and drop.http://imgur.com/
You can upload your picture by drag and drop or by searching your computer. When uploaded, select "BB code forums" and size.
Copy the resulting code and paste it on your draft post. Click on "preview" to see what it looks like.Here's an example of a "large thumbnail".
After experimenting around you should be able to get the size you want.