I have an idea for this, but I don't know where to make the picture, how to get a url for it, and how to make it 100x100.
So in other words your a complete noob at this OK...
You can make the art using one of tons of programs including MS Paint which comes with all windows Operating Systems, but a good one would be GIMP or Paint.net( a couple others are corel paint shop pro, and PhotoShop,but PhotoShop costs a good amount of money), all depending on what you are looking for and which program you find to be easy to use.Here are a few links.
http://www.gimp.org/
http://www.paint.net/
Now then as for where to get the URL, when you save your image that you made, you need to put it on a free image hosting site.From here you need to grab up the direct URL of the image(which the URL should end with a image format, such as .png or .jpg at the end of it depending on what format you saved the image as) Here is a link to one popular and reliable free image hosting site called Photo bucket.
http://photobucket.com/
As for your last question, when you open up a new image document I know GIMP allows you to set the parameters,but if you have issues with any of this,just contact me on my profile, or use the tutorials thread in this forum section.