http://www.flickr.com/services/api/misc.urls.html

Photo Source URLs

You can construct the source URL to a photo once you know its ID, server ID, farm ID and secret, as returned by many API methods.

The URL takes the following format:

http://farm{farm-id}.static.flickr.com/{server-id}/{id}_{secret}.jpg
or
http://farm{farm-id}.static.flickr.com/{server-id}/{id}_{secret}_[mstb].jpg
or
http://farm{farm-id}.static.flickr.com/{server-id}/{id}_{o-secret}_o.(jpg|gif|png)

Size Suffixes

The letter suffixes are as follows:
s small square 75×75
t thumbnail, 100 on longest side
m small, 240 on longest side
- medium, 500 on longest side
b large, 1024 on longest side (only exists for very large original images)
o original image, either a jpg, gif or png, depending on source format

Note: Original photos behave a little differently. They have their own secret (called originalsecret in responses) and a variable file extension (called originalformat in responses). These values are returned via the API only when the caller has permission to view the original size (based on a user preference and various other criteria). The values are returned by the flickr.photos.getInfo method and by any method that returns a list of photos and allows an extras parameter (with a value of original_format), such as flickr.photos.search. The flickr.photos.getSizes method, as always, will return the full original URL where permissions allow.