How do I change the sharepoint logo to add a picture and "welcome" Message? Is this done in Web parts?
No. It is in Site Settings >> Title,Description,andIcon.
Upload an image to a library on the site. Copy the URL of the image. Go to the above location on the site. Paste the URL of the image field and save. Be mindful of image (not file) size. Too big will drastically change your page look.