Flickr only store a reference to an image in their databases, the actual file is stored on a separate storage server elsewhere on the network.
A typical URL for a Flickr image looks like this:
If we split this up we get:
farm1 - Obviously the farm at which the image is stored. I have yet to see a value other than one.
.static.flickr.com - Fairly self explanitory.
/104 - The server ID number.
/301293250 - The image ID.
_dc284905d0 - The image ‘secret’. I assume this is to prevent images being copied without first getting the information from the API.
_m - The size of the image. In this case the ‘m’ denotes medium, but this can be small, thumb etc. For the standard image size there is no size of this form in the URL.