Often times, you'll upload a decal with a transparent portion, and when you go to use it, you'll see something like this:
An outline appears out of nowhere to make your paint.net editing skills look bad.
You know the image should really look like this:
This isn't a problem with the image, it's a problem with the way ROBLOX samples their images.
If the size of the image is native and the rotation is 0, you will see no border, but if you want to have rotations and different sizes, you'll have to do something a little more complicated. This is the solution I use when I have this problem.
I wrote some Lua code to help combat this a little. It works pretty well as you can tell by the example pictures above. You can download the stuff below, and watch the tutorial on how to use it below.
You can get pixelformer here: http://www.qualibyte.com/pixelformer/download.html
If anyone knows of a PNG library for Lua, I'll happily use that and then it will be a one step process.