Twitter recently made it so that any tweeted image gets a ton of ugly JPEG compression added to it... except in certain cases!
The exception is, if you upload a picture with transparency in it, Twitter will store it as a PNG instead of a JPEG in order to preserve that transparency... which incidentally means that it won't be able to JPEG-compress it!
To that end, this app takes any image you drop on it, and gives it back to you as a PNG with one pixel at 99% opacity, which is enough for Twitter to consider it "a transparent image" (while making basically no visible difference to the picture). Then you can post that 'twitter-safe' copy onto twitter and spare yourself the compression!
(One note: unfortunately this ONLY works for images on tweets -- it won't work for your icon or your header.)
Note that this app does not transmit anything to any servers -- the conversion happens completely in your web browser; the image never leaves your computer. (Until you then go and stick it on twitter afterwards, I mean. :P)