Here is how to make a PNG. If you have any questions feel free to ask in the comments and I'll be happy to help :)

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1) Find an image that you want to cut out, I'm using one of Phoebe Tonkin. I found this image on Google.

2) Click on the quick selection tool (in the red). Make sure that the size (in the blue) of it isn't too big because then you would select the background which is what you don't want.

3) Go on your model and make sure you select everything you don't want to accidentally cut out her hand. It's okay if it selects some of the background because you can erase that later. 

4) Once you've selected everything double click on the picture, make sure you are still on the quick select tool, and press "select inverse" this should select the background now. Then just press delete. 

5) Now pick the eraser tool (in the red) and set the size pretty small and the hardness to a 100. Start carefully going around the model and smoothing out the edges and removing any excess of the background. I recommend zooming in and doing this because it will make your life much easier. 

TIP: I would recommend reselecting the model using the quick select tool then pressing "select and mask" which is going to be on the top. You can feather the edges and smooth it out after erasing to make sure everything is completely smooth.