In GIMP, once you've got a background layer that supports transparency - which can be accomplished simply by duplicating the background and deleting the original, in most cases - you can use the "Magic Wand" tool to select the area you'd like to be transparent. Making sure that you have selected only the areas you're okay with erasing, it's just a matter of deleting the selection, which should reveal a checkerboard pattern behind it - indicating transparency. You can then save the image in a format the supports transparency (such as PNG of GIF), and you're done.
Q. Now that I have a bunch of stuff made, where to put it?
A. There's loads of places to upload stuff on the internet, depending on what it is you made. Various art and HTML sites support dollmakers and flash animations. Photobucket is a great place for all your miscellanious stuff, and it can hold videos, too. Too bad it can take a REALLY long time to upload larger pictures (it took me about 15 minutes to upload 3 1200-2000 pictures!).
Links
Click here or for a fully customizable dollmaker, click here
EchoEcho.com The link doesn't work anymore, but I'm leaving it up just in case it starts up again.
HTML Goodies A great overall site for HTML tutorials.
Doll Tutorials
A great tutorial site for dollmakers depending on what programs you have. This tutorial is program specific for Flash, I believe; I haven't looked in a while.
GIMP Help Files (Non-downloadable) These are VERY convenient for anyone who has GIMP (though I intensely dislike GIMP for a number of reasons).
Basic HTML Help Even though it's part of Neopets, it's still a very good page for rudimentary HTML. At one point, it even had a listing of all the HTML colors available, but you have to have an account to use it. It's under NeoAdventures somewhere I believe. (Later on, someone alerted me of another one; on Wikipedia with even more colors)
SurveyMonkey Ignore the weird name. This is an excellent site for creating...surveys.
Quiz Script
A neat quiz script, different from what we use here, but worth checking out.
ʇxǝʇ uʍop-ǝp�sdn A very quick way to make upside-down text. Unfortunately, it only uses lowercase letters.