First erase the white with your magic eraser.  Next, we will give it just a bit of a black outline using edit-stroke (outline) selection.  5 pixels worked just fine for me.

Next, we want to give it a nice white outline to give it that sticker look.  Create a new layer.  Then hold the control key and click on the layer of your sticker over in the right hand column. This will select all of your sticker.  Head to your top bar for select--modify--expand and play around with the pixels that work for you.  Here I used 15.

Make sure you have the new layer activated and pour white using the bucket tool.  It will make an exact copy of your shape--only larger by your choice of pixels.  Move the layer of white beneath your colored layer and merge.  Here I added a bit of shadow so you could better see the sticker.

You can use brushes to create shapes to start with just as easily.  Remember to follow all copyright rules if you are creating kits for sale as they are different than personal use rules. 

Want another layer of fun for your sticker?  Head over to atomiccupcake.com and purchase the epoxy action.  With just a click of a button, you can add that extra layer of shine to your sticker.

There you have it--an easy way to make stickers--and a technique you will find very helpful in your scrapping.  Makes great outlines for photos, titles, and so much more.