Information on how to choose the display size of an image may be found in the following thread.
http://thefiringline.com/forums/showthread.php?t=554362
If you had an image that you were going to display as follows:
[IMG]http://www.imageurlgoeshere.com/myimage.jpg[/IMG]
and it turned out to be larger than would easily display well in a thread, you could adjust the image size to something more manageable by adding the [resize=X]...[/resize] as follows:
[resize=700][IMG]http://www.imageurlgoeshere.com/myimage.jpg[/IMG][/resize]