For image posting you need an address of the image to copy and that means going to a site that has a URL address. Using your own photos on your desktop or hard drive won't work.
I use imgpile to hold/store my pictures, photos and what have you, using their image address. The site is free and easy enough to figure out. Once there, all you need to do is right click the image, select copy image address, and use that URL to post to the image icon (URL address) from above.
After that, you can fiddle with the size until you're satisfied and then post.
All the best,
Nonoise