There's a little app that I've used in the past that can resize them, I think downsizing to something like 160 x 120 comes up ok, but trial and error will see you right and you can always preview your post to check it.
http://pixresizer.en.softonic.com/
its ok, but maybe someone has a better one?
If using an image hosting site (photobucket etc) copy the IMG code that's generated and paste it directly in you post, make sure you don't reorganise your albums after you start posting though as this destroys all links.