Pixlr won't actually make transparent gifs. I make mine transparent on Online Image Editor
There are 2 options:
If your doll is just a transparent PNG (TPNG on the TDP maker) then when you upload it you can choose the convert image option and choose GIF then upload it. Then you can just save it from there.
I will use pixlr to make the doll a png, using an odd color on all of the parts that I want to make transparent. Upload the doll. (if you use the glitters on this site you will need to do them now) Click the tab the says Wizards then the Transparency option. For the options you will choose Select Transparent Area then set the fill type to Replace Selected Color then just click the odd color on you doll. Then you can save. I have the settings below.
You actually can make a doll transparent on Iaza as well.
You'll need to make the doll with the odd color for the transparent areas. You will need to know the Web Color code of the color though. When you upload on Iaza choose the option on the top right that says Transparent. Now if you know the color number you can actually type it in. Or click click Color to bring up the swatches. I tend to use a really bright colors. If you are using paint it will be one of the basic colors in Pixlr it will be one of the Web Safe colors. (I usually use the bright aqua.) If you use one of those you should be able to pick the color from the swatches. However you do it when you have the correct color you will click the Convert button. (if you are picking the color instead of using the number you might need a couple of trys. So if it doesn't work click the arrow to the left of the image to undo your changes) After your convert it the background will look white, but it's not. Then click on the little link under the image that says View Image. That will open a new window, which will show your image correctly, then you can right-click and save the image. Refer to the image below.
I personally prefer Online Image editor because I don't have to know the color number.
Anyways I hope this helps. Let me know if I need to clarify this.