Paintbrush Alternative

Hello!
On my Mac side of my MBA, I run this nifty little app known as Paintbrush. I use it to make sprites and Minecraft textures.
However, I don't want to have to reboot in to a different OS, nor do I want something like VirtualBox or Parallels to use it. I want to be able to use a similar app on the Windows side of my laptop, where most of my gaming things stay.
What's the closest alternative to this? Paint is very similar, but lacks a core feature that I need - transparency. I need to be able to give an image a transparent background, or wherever the space that the sprite doesn't use is.
I also need features like the "Image Size" and "Canvas Size" shown in Paintbrush. I use these commonly as well, combined with a decent "copy-paste" function and ability to work with .PNG files.
I will link Paintbrush here so you can check it out for yourself:
Paintbrush on SourceForge

Thank you for your time and patience, I really appreciate it!
-MarioMover

Are you willing to spend $$ for a good Windows program or are you looking for freeware?

Freeware, though at the time of writing, Paint.NET seems good enough for me.

Gimp. Every time. It is freeware, and you can have a native OSX version as well as a Windows version. I have found it to be better than Paint.NET