Those who work in a creative field, whether it's as a 3D modeler, or a web designer, usually end up with tons of images to deal with. These could be textures, resource files, projects you're working on, etc. If you dump them all in a single folder, it doesn't take long until you have thousands of files, and you can't find what you're looking for anymore. Here's a few ways you can effectively organize your images.
Google Picasa is actually a pretty decent option for keeping track of images. It's free, which is always a plus, and when you first install it, the program can look around and find all the images you already have on your system, which is a big plus when you already have lots of unorganized files. The default view shows you the images by folder, and you can easily add a description to each folder right there in the interface.
When you click on an image, you get properties like size and resolution right there on the right panel, and a search box at the top allows you to search by file name. Also, Picasa includes a sync function to the web version of the program, so this helps you keep a backup of all your images in the cloud, or if you have to work on two computers (work and home) then you can always be up to date on both systems.
Folders and metadata
Another way to work, if you would rather not use a specialized program, is simply using folders. If you're in Windows 7, you'll see that the Explorer interface is actually much better than before. If you right click on a file and choose properties, you can view all the metadata from the file. You can also add a name, title, comment, and so on to any file you want. This makes the search function even better than before. For example, you could add a comment to a file, and then when you search for a keyword across your system, it will take into account that comment.
A lot of people love Lightroom for managing photos, so it's worth including it here. Certainly, it has a lot of advanced features for organizing and editing files. However, there are drawbacks, mainly the fact that it isn't free, and also it's really made for photos, not images. There's a lot of popular file formats it doesn't see, for example.
Overall, it's up to you to decide how you want to organize your files, but if you work in the graphics industry, or even if it's just a hobby for you, then you should use some method, before ending up with thousands of images you can't make sense of anymore.