A profile picture is an essential part of your account! It's visible to yourself, your conversation partner, and also to your followers on the social media feed on the main page. You can change that photo on your Profile tab from the left sidebar menu.
You can take a picture from your device by clicking on Upload Photo > Camera > allow your browser to use your camera if needed > click on Capture.
If the site won't let you upload your desired picture, try resizing it to a smaller format. The square dimensions are the best to prevent it from appearing pixelated. A photo that isn't uploading might also be in a non-traditional format (standard options would include .png and .jpeg), so try a more orthodox file type.