To change your profile (avatar) photo that appears on your profile and with your posts, click in the upper right corner where you see Your Houzz. From there, select Edit Profile. Once on your edit screen, click your current profile photo where you see thecamera icon.
Once on the Edit screen, you will see your current photo (if you have one) on the left side of the screen. To upload a new photo, click the PLUS sign on the right to browse your system for your photo.
Once you are happy with the photo, click Save to keep it as your new avatar.