Step 1. Download Gimp here
Step 2. After the installation is completed find the folder you store all of your pics in, then select the one you want to render, i'll use this for reference.
(Optional). You may want to put on some music because depending on the size of the pic or the amount of shit you'll have to render out, this could take anywhere between 25 minutes to 3 hours.
Step 3. When you open your pic zoom in to 400%, then click on the free select tool (looks like a lasso) and find a spot, any spot, on the render and start from there.
Step 4. Now, to get the clean effect you render the character itself ( like this) not the outline.
Step 5. Continue doing step 4 until you complete the outer part of the render. Then Apply step 4 to the characters hair or any other part of the characters body that may still have some background next to it.
Step 6. Fell free to render out any parts of of the picture that just shouldn't be there, like that pink thing on Vanessa's left arm.
In the end you render should look like this
(i know, i know, the left side of her hasn't been rendered properly)