How do I change my company's description or picture?

On your job's overview page, click "Job Posting" on the toolbar near the top.  Once you see the job post you've written up, click "Edit" under job's name.

Right under your companies name and description, you'll see two options: "New Company," and "Edit this company."  Click on "Edit this company," and you'll be brought to a field where you can alter the description, and a place to upload a picture on the right.