When you make your post, if you scroll further down the page you'll see a section called "Additional Options". In that section click on "Manage Attachments" and in the box that pops up you can browse your computer files and select the image you want to upload.
Note that you need to stay within the site's file size and image size guidelines. If you need to resize, use a free program such as irfanview to do this prior to uploading your images.
