If you already have your image uploaded on the web it's quite easy. Locate the button on the posting toolbar that is a picture of a tree (when you hover over it it should say Image). Click that, paste in the URL of your image, click Ok and you're done!
If you don't already have your image uploaded on the web then click the button that has a picture of a tree with a disk on it (when you hover over it it should say Host an image). Click browse, locate the image you want to upload and click Open. Then edit your resizing preferences, click Host it and you're good to go!