To expand a bit on what Marc has noted, generally to have download capability, you often need an active support contract for the device. (An exception to this, is for updates to address some major security flaw. Then, if you contact Cisco, they often will allow you to download a version that fixes that flaw. However, you might have to "prove" you're the "original" purchaser of the device.)
Further, depending on the device, you may see "different" feature set images. On older devices, you (legally) need to insure you only upgrade to a feature set image you're licensed for. (Newer devices, often now have a built-in licensing feature, and for those, you often just load a "universal" image.)
Lastly, there is often many releases to chose from. So another consideration is which one to use. Generally, you'll may want to use the oldest release "train" that supports the features you need, using the latest release in that train. (This often helps insure that release's stability.) Sometimes/often Cisco will "tag" recommended versions, again for stability.
