When you create a new thread, farther down the page than the posting box is an area called "additional options." In that area, there should be something called "Manage Attachments." Here's a screenshot of where it should appear on the page.
Click on it and a popup should come up. If you have a popup blocker, it maybe stopping it.
On the far right should be a tab with a green button that says "Add files." Click on it.
Another box will appear and the top blue line will have a link to computer or website. Computer is the default. Then at the bottom it says "select files" click on that.
It will bring up the box to browse your computer and select the file.
Once it is selected, click upload file.
In the original popup, it should show that a file has been uploaded. Select it, and then click the "Insert Inline" at the very bottom, right of the popup. It may be too big to fit on the screen, so you may have to resize that window, or scroll down to it. The picture doesn't show up in the message box, but it will have a note about an attachment, and then when you preview your post it should show up.