At the bottom of the submit post form is a check box to add an attachment. Check this and then click "post". The next screen you'll see will direct you to search for the file on your computer. Once selected click "upload" and you're good to go.
Remember to downsize the photo for the web - under 300K is good, otherwise it may not upload (plus it adds to WL.com's storage costs). Send us a Feedback if you're having troubles with this.
I use this to resize pictures - it works great, is fast, and can do batches. It makes new copies resized and leaves the originals alone. And it's free.
MIcrosoft's Image Resizer
Editor,
COOWashingtonLakes.com