Well, for the avatar you can store it on Flickr or Imageshack (or just about any other hosting site). Just make sure the link you give it is just http and not https
As for the files, you can upload jpeg files that are under 200kb size (I make mine 198kb because I discovered that some programs cut corners and show 200kb and the server then refuses them).
Just click on Attachments and other options right under here ↓ (when you are writing a reply)
Browse to get the file, click (more attachments) to put another file.