There are two options...

1/ Open an account on an image hosting site like Photobucket, imageshack etc. then hotlink images from there.

2/ When you hit "Post Reply" (not Quick Reply) scroll down to "Additional Options." You'll see a section called "Attach Files." Click on the "Manage Attachments" button and a small window will pop up. In that window you can click the "Browse" button and find the file on your computer to upload then click "Upload." Alternatively you can also upload from a URL by copy and pasting the URL into the "Upload File From a URL" bar and then clicking the "Upload" button.

Hope that helps.