For your first question, go to User CP up in the top left of the forums, then the link "Edit Profile Picture". Click on the button "Choose File" and choose a photo from your computer.
For the 2nd question, downloading photos in gallery. Not sure what you mean here. Perhaps you mean uploading? As in putting some of your photos from your computer into the gallery? If so, click on "Gallery" along the top, then "Upload Photos" link on the right hand side and follow the prompts.