What Mike Pierce said in the comments above is your answer; Craft doesn't create the image transforms until you ask for them.
So for instance, in a index.twig
file you might do something like:
<img src="{{ entry.someAsset.getUrl() }}" />
...which would output the image. If instead you wanted to output the image transformed based on one of the transforms you made in the AdminCP, you'd do:
<img src="{{ entry.someAsset.getUrl("someTransform") }}" />
...where someTransform
is the handle to the transform you created in the AdminCP backend. Craft will then perform the image transform, and it'll output the transformed image on the frontend.
The next time that the same transform on the same image is requested, rather than doing the transform all over again, it'll just use the already transformed image that it's saved to disk.