Android/iOS: Twitter has released an update to its mobile apps that brings photo filters to its image service. You'll also find new photo-editing abilities.
To edit a photo, click the image icon, select the picture you want to use (or take a new one), and you'll get the edit photo page. Here you can apply one of eight filters, crop the image or run an auto-enhancer that adjusts the balance. The Android version is available right now, and the iPhone version should be out shortly.