I am unable to find the "browse" button so that I can post a pic.
Under the help button you find instructions on how to post a picture.
Specific to an existing topic, such as the weekly Challenges ("contests" per your title), once you're at the specific topic, you need to click a REPLY button. Then, below the text field that appears you'll see the area to load a photo.
The link to "how to" that Dave references is:
https://www.uglyhedgehog.com/help/how_to_post_a_picture.jsp.
If you want to reply, then
register here. Registration is free and your account is created instantly, so you can post right away.