When I first joined KB, before I posted a picture, I read the tutorial on how to post images. (posted below)
I mistook a tutorial suggestion of posting images at a width of 600 as the maximum allowed. It seemed to fit without causing a scrollbar at the bottom of the posts frame, so I continued to go with that. As a moderator, when I see a post with a scroll bar, I'll adjust the image to 600 wide. Nothing wrong with it, but I'll admit to instructing people incorrectly for the last 8 months.
So,
today I learned that 629 or less is the maximum width a picture can be posted without a scrollbar appearing. Or, just 625 for an easier number to remember, and a number I see many people using.

*ADVANCED IMAGE ADJUSTING TIP*
If you pic is too big or small, you can add a displayment size adjustment to the image tag by adding information like this:
Image Tag: [img]
Change it like this: [img width=600]
And that's it... it's that easy!
HAPPY POSTING!
~Gia1978