Paint.NET
inb4jdsaysgimp
Haha, not if I say it first.
I've been using it for over 3 years for everything from schoolwork to model textures, and it's a pretty decent program.
A lot of times, when I see people make stuff with Paint.NET, they seem to forget to add in anti-aliasing. I don't know why. I've never used the program before. I'll assume brushes don't have blurred edges on by default. Everything I've made in GIMP has had properly smoothed edges.
Now I'm going to totally blow your mind and contradict myself- use Inkscape instead. It makes great looking decals. GIMP and Paint.net are great for photo editing. If you want to make crisp decals that you can easily resize without have to worry about them becoming blurry or pixelated, use a scalable vector drawing program. You can post the .svg file for other people to use to make edits of your decal - and they will be of an equal quality to your decal. If you just give someone a lovely .png to work with, chances are they'll make it blurrier or screw up some colors or leave behind artifacts and it will suck.
Here's an example scenerio. I post a guy with grey hair. Someone wants purple hair. They open it up in photoshop and use the magic stick or whatever adobe call it to pick out all the white stuff and color it. Now either they made it too sensitive, and you end up with a series of white dots around the border because the anti-aliasing made that white gray, OR they make it too unspecific, and they end up selecting a bit of black too and turn it a stupid shade of dark purple.
Now if you took the time to do this stuff in Inkscape, all they'd do is grab your .svg, click the hair line, pick a new color, and export it. And there you have it, beautiful purple hair as perfect as your initial decal.
Finally,
the vector art style fits in perfectly with the existing blockland decal style.
Pictures of telephones you got off of google images and then added transparency to in paint.net or GIMP
do not.
Now I'll contradict myself again. If you're going for certain effects (sketchy or hand-drawn for example) effect, do it in GIMP/Paint.NET/Adobe Photoshop (I'd recommend just buying Photo Elements 4.0 instead, it will do what you need and it cost about the same as a new computer game). You might also want them to add certain special effects (you will definitely want to emboss or bevel some stuff, vector programs don't handle that stuff).
TLDR VERSION
GET THIS PROGRAM INSTEAD OF GIMP OR PAINT.NET. IT'S ACTUALLY MADE SPECIFICALLY FOR WHAT YOU WANT TO DO, INSTEAD OF AS A GENERAL PURPOSE IMAGE EDITOR.
