The Amazon Kindle has a 4-bit grey scale 800x600 screen. With the right tools, you can change the images it displays for the screen saver to any 800x600 PNG or JPEG image, but you have to do some work to get them to work right.
In GIMP, I either split them into RGB or desaturated them (depending on the image), tweaked the color levels, then converted them to a custom pallet with 16 shades of gray with Floyd-Steinberg dithering.
The results look pretty good on the Kindle, and I imagine they'll work just fine on other eBook readers.
1. Go to Applications folder and find the Twitter.app 2. Right click and click in Show Package Contents 3. Browse in /Contents/Resources/ 4. Drag and drop the Icon.icns that you downloaded into this folder and replace him. (Recommended: backup the original Icon dragging the icon into another backup folder)
Since Twitter for OS X has (apparently) lost the support of Twitter, and hasn't been updated with the new Twitter bird, I have created one. It is designed to fit in with the other stock OS X icons taking (a little) inspiration from the new iTunes icon (and the new App Store icon I created). If Apple made the Twitter app, this is what the icon would be like