Modern browsers all resize images to fit the window on the fly (ie6+, firefox 1.5+). I think you will find that over 85% of the forum's viewers are using browsers that do that. To embed outside images would be potentially more harmful because without the forum restricting max resolution, what's to stop someone from embedding an image even larger than 1024x768? An image that is being embedded into a page that is already taking up some screen area, and an image that does not get resized on the fly by modern browsers. I've seen threads here where you have to scroll horizontally because posters embedded huge images into their posts - that instance is far worse than what you are concerned about IMHO.
But even an increase to 800x600 would be great.
I don't mean to come across as argumentative or rude. Just trying to make the forum better. :)
The bots can be controlled to a degree btw. Basically you need to setup stripped down sections of the forum and use the robots.txt file to restrict what they access. Have a look at how RCGroups handles them - they run similar forum software. More info on robots.txt can be read
here.