If you right click your image there'll show up a menu of options, click the one called Properties near the bottom. Copy the image adress (url) and paste it between [img] tags like these:
- Code: Select all
[img][/img]
(You'll find the [img] tags when typing a post, they are one of the buttons above the writing area).
Submit or preview and the image should show up.
You can also click Save image as... and save it to your computer. Then you upload your image to an image hosting site like photobucket, tinypic or iaza. By uploading the image yourself you can chose the size it will be, which is good since oekaki images are a little big to fit into signatures^^.