First of all: Welcome to the forum! I consider myself a beginner too but I think I can answer your questions anyway.
1. Yes, you need to host your images on a hosting site before you can post them in the forum. I use Imageshack
http://www.imageshack.us/ myself but some prefer Photobucket.
2. Read a good tutorial (check out st0vens site:
http://www.spriteart.com) Then begin to study some of your favourite images/sprites. Then pixel something on your own. Then study some more. Then pixel some more....
3. I don't think it matters what you do as long as you start with something small. That will help you to develop you pixeling techniques.
4. I've never used this program, so I don't know if it's good or not but I suggest you download Cosmigo Pro Motion. It may take some time to get use to all the keyboard shortcuts but ones you've learn them it will be a wonderful tool to work with.