The picture must be put on a photo-hosting website like Photobucket. There are others. It's free.
Then, when you upload the picture from your computer to the website, it will give you the link (also called a URL) you will use in your posting here on DU. You can tell if you did it right if you then use the preview button below where you are typing your post. If you did it correctly, you will see the picture.
If the photo is on a website and you want to share it here, just right click the photo and you should see something like 'copy image url'. Then paste into the posting on DU.
This is an example of what you will use as a link. It will look something like this (but so I can show you this link instead of the photo it points to, I'm intentionally putting in some spaces.