If you see this soon, you will still be within the edit window of your original post- about 3 hours from the original post time. Click edit, then click go advanced, then click on the attachment icon- the paper clip at the top of the edit window, then another box will open which will let you browse your pc. Select the file or files and upload. Then go to preview to see how it looks, then save changes.
If you are beyond the edit window, make a new post to this thread and use the advanced option which will have the attachment feature as above.
David