No need to change them, if you use the thumbnail option of an image hosting site.
Example:
[url]http://picturedumper.com[/url]
The result:
If you want to post large (hotlinked) pics, you can reduce the size of images with several programs, e.g. IrfanView.
It's free and easy to use.
http://irfanview.com/
Monkey D. Ruffy