@rdh61,
you will need the image's dimensions, which you can determine from its properties.
for example...
this image is 1600 x 1200 -- way too big.
Code:[img]http://wallpaperswiki.org/wallpapers/2012/10/Grass-Field-Painting-Artistic-Creation-Design--1200x1600.jpg[/img]
to reduce it so it fits nicely in the post window, i can reduce the image by halving the dimensions like so --
Code:[img width=800 height=600]http://wallpaperswiki.org/wallpapers/2012/10/Grass-Field-Painting-Artistic-Creation-Design--1200x1600.jpg[/img]
always preview your post to make sure the image is both visible and the size you would like it to be...