For the pictures that are too big for the forum, you can use a software to reduce/convert them, such as
IrfanView (free), or if you really insist on posting a big size picture, you can use for instance
ImageShack : a thumbnail linking to the full-size picture will be displayed in your message, but this is not recommended because often links get broken after a while.
(here, a berberis that was displayed at our local exhibition in September)