All downloads on this page are free for use for any purposes, as long as it's awesome. If you do make use of them for public consumption, a mention of Rebrickable would be nice.
Click on a link to download all available parts images for that color. All of these parts images are 100x75 PNG files. There is also a collection of larger parts images (500x375) with each part appearing in a single color only. These files are automatically updated at the start of each month.
The inventories of every official LEGO set in the database is available as a single gzipped CSV file here. You can also get the list of parts and the list of colors used by the inventories. These files are automatically generated at the start of each month. If you need it to be more accurate, use the API which provides real time data.