For what it's worth regarding dimensions for graphics, my recollection is that on some pages the maximum width is something like 564 pixels. Going over that could make them disappear. I know I made some of mine just under that at 560 pixels to be safe. It may depend on which part of the site you are on. They don't all use the same formatting or style sheets. Staff can correct me if things are different or my memory is failing.
Going over this width may cause cut-off or even, I believe, in some browsers on some pages of the site, the picture not to display at all. I think I'm right on that. Reduced browser width will definitely cause cut-off.
But while 564 pixels may be the maximum I would recommend staying well under that if possible. Don't forget to consider that while members use various screen resolutions, they also may not be running their browser full screen. I know I don't, plus I use a side-tabbed browser, further cutting down view window width. The advantages I get for this in the forums and on the SHH sites are tremendous so well worth the width trade-off.
I recommend doing some browsing in the tutorials and walk-throughs with your browser not at full screen to get a feel for width sizing and cut-off by looking at currently displayed screen shots.
While you can right click and choose "properties" to see the displayed size of graphics in many browsers, I sometimes use a handy little tool (there may be others) for measurements called Screen Calipers. It's shareware and stated that the trial version may be evaluated for as long as you need to at
www.iconico.com.