YankeeJim: Did those pics above take a while to load?
No.
YankeeJim: And whatever size the actual picture is, [...] will it always fit when you put 100% in?
This is where it gets a little complicated.
When you insert an image using the Insert Media icon, the height and width values that you can enter, set the MAXIMUM allowable width and height for the image (if these values are left at '0' or deleted, the image will post at its default size.
So, if you change the width value in the Insert Media dialogue to 450 upon posting, your image will post at a width of 450 pixels if its natural width is equal to or greater than 450, because you have set the MAXIMUM width to 450. If your image is less than 450 pixels wide, it will post at it's original size.
The Edit Image dialogue box allows you to a specify a width and/or height for your image, but it is governed by the MAXIMUM width and/or height settings you specify in the Insert Media dialogue. Putting a width of 100% in the Edit Image dialogue, will set the picture's width to either the maximum width of its containing element (in this case, the width of the forum) or the maximum width specified in the Insert Media dialogue, whichever is the smallest.
If you enter just a number in the Edit Image dialogue and not a % figure (it will assume pixels in that case) it is all too easy to make the image too big to fit where you are posting it. Inserting an image into a wall post with the Insert Media dialogue still gives you a default width setting of 550 (it should be about 250), but if you edit the image using the Edit Image dialogue and insert a width value of 100%, the image will be reduced to fit the width of the wall post so you can see all the image.
I'm not sure I've made it any clearer, but I tried.