To access the setting for your personal avatar (the photo to the right of your post), click on the "User Control Panel" on the top right of the page. When you get to the UCP, you will see a tab called "Profile". Click on that and you will see a setting for "edit avatar", click on that link. Here you will be able to upload a photo from your computer as your avatar. The maximum size is 90 pixels by 90 pixels and 976 kB. To upload, click the box that says "choose file" and select the image from your computer. You may also link to your avatar if it is already on the web by typing in the URL of the image in the "Upload from a URL" text field. After you have selected the image, press the submit button to complete the avatar change request.
To change your signature (the text that follows each post you make) go to the "User Control Panel" and click on "Profile" again. Then click on the link to "edit signature". Now enter the text for your signature. It is usually helpful to have your name and your affiliation so that you don't have to type it in for every post. Text formatting is also available for signatures. Once you have completed your new signature, press the submit button to complete the process.