After taking the picture, and loading it to your computer you'll need to host it online. You can do this at imageshack.us or photobucket.com. While there are several others, those happen to be very popular. Then you link. It should give you something to copy and paste such as

all you need to do is put that into your post and you're good to go!

Now, to put it in your signature, take the same image url (With tags) and go to Profile > Signature.

Just remember keep it in reasonable size