You can add relatively small images as attachments to a message. You will have to use the full message editor:
You can then find them in the Attachment section of your user profile of the forum:
Paste the link to the files in [img] [/ img] tags, or use the button on the text editor box.
[edit: Darn, needs to be wide images. Tall images like mine get squeezed.
Alternatively, for higher resolutions you can also create a new album (higher up the list in the last image) and upload images to that too. Then place links to them in your message.