larger file sizes make larger pictures. if you start with a full resolution file, when you go to upload it you'll have a couple of choices. Choose the largest size available and when it uploads you can see if you like that or want to go smaller.
It is all in how you link the picture to the message board. If you upload a picture to your personal folder on the message board then click on it and copy the url then open your forum post and add the picture by referencing to the URL it will then be full size.
There may be a more elegant way but that will do it.