Undersampling
Your sat in a tour bus in the Serengetti looking out over the vista to a herd of Zebra, get your digital camera out, click and capture that image, driving away you capture some more of the same zebra. The original is effectively infinite in detail but your camera will capture a number of pixels depending on the camera resolution, encode them (probably compress them too) and store them on the camera flash card. From that piont on you cannot get any detail from that image with a finer resolution than that of the cameras imaging sensor. The camera sensor has 'sampled' the infinite detailed image at regular intervals and collected only those samples in the camera.
Later on you look at the zebra and note that in the middle picture the stripes on his front leg are reversed to the first picture and then later, wham !, what the hell happened with that third image ?
What your looking at are the effects of undersampling and a process called 'aliasing'. The same effect is also called Moire when referred to 'fringes' that appear in closely spaced lines at a changing angle. An alias is a second instance of something usually a second name of a gunslinger like "Alias Smith and Jones" (http://www.imdb.com/title/tt0066625/). In graphics they refer to secondary instances of stuff which is not supposed to be there - like the new zebra stripes. Also, once you have them it is extremely difficult to recover the image so that it shows what the stripes should have been like.
The precise nature of what is happening to your zebra leg is quite complicated and is wrapped up in a lot of math which we must lightly dip our toes into in order to fix the problem. Suffice it to say that in this instance what happened was that the next sample skipped over a stripe and went into the next one after. Its the same as the wheels going backwards on old cowboy movies of the wagons rolling where the film camera sample rate of 24 frames per second is just below that of the wheel spokes moving around. The movie camera frame rate is undersampling the action.
This is the main reason why holding original photos and art at high resolution is essential. If your going to work on a final piece of art at 1000x1000 pixels then you should work in much more than that - say 4000x4000 and only convert to 1000x1000 at the final stages. If for any reason there are details in the original that are too fine to produce at 1000x1000 then they will not only be lost but could cause weird effects to be produced at the final re-sample.
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