Software to create bitmaps for GUI toolbar buttons

Question

Can anyone recommend software that facilitates the creation of bitmaps for GUI (Graphical User Interface) toolbar buttons?

Requirements:

1. Facilitates creating multiple states for a button, including enabled and disabled
2. Facilitates creating multiple sizes of the same image (eg. 16x16, 32x32, 48x48)
3. Supports alpha (transparent) pixels
4. Freeware (gratis)
5. Runs in Windows

Preferences (not required):

1. Supports gradients
2. Supports geometrics (eg. rectangles, ovals, triangles)
3. Facilitates generating shadows of objects in the images
4. Portable software
5. Currently maintained (not abandonware)
6. Open-source

Answer 1

Gimp:

Requirements:

1. Facilitates creating multiple states for a button, including enabled and disabled - Has a "New Simple Beveled button" file entry, if you can come up with a simple rule for disabled state then you can script it.
2. Facilitates creating multiple sizes of the same image (eg. 16x16, 32x32, 48x48) - I would recommend always creating your biggest resolution first and down scaling to the others - you can even add a script to generate all the sizes and save them
3. Supports alpha (transparent) pixels Yes - depending on the file format selected
4. Freeware (gratis) - Free and Open Source
5. Runs in Windows (any version) - Yes you may need an older version to run on Win3.1 don't think it will run on Win2 but not much did - also runs on Linux/OSX/more

Preferences (not required):

1. Supports gradients Yes
2. Supports geometrics (eg. rectangles, ovals, triangles) Not directly did I mention scripting in Script-Fu or Python
3. Facilitates generating shadows of objects in the images Yes
4. Portable software Yes e.g.: from Portable Apps
5. Currently maintained (not abandonware) Very Actively
6. Open-source Yes