Took me a while to find this information and figure it out, so I though it was worth posting my findings here.
Basically I have a Nikon D40 which is a 6 megapixel digital camera with a sensor sensitivity of only 200 ISO - it's a great camera but I've recently become a contributer to iStockphoto and have had some files rejected because of artifacting and noise issues:
This file contains artifacting when viewed at full size. This technical issue is commonly created by the quality settings in-camera, in post-processing, in RAW settings or scanner settings. Artifacting can also be introduced into an image from the result of other factors such as excessive level adjustments.
Noise (pixels of varying color where there shouldn't be) is most commonly created by digital cameras, especially in darker shadows or under low-light conditions and exacerbates the compression issues mentioned above. You might want to double-check to make sure that your camera's ISO/ASA setting is at the lowest number (usually 100). In digital cameras, higher numbers (200 or 400) will always result in more noise (just as with film).
Hopefully you can make out the 'noise' in this sample - enlarged by 200% - from a rejected image
As I already shoot in RAW settings and use minimal post-processing in the latest version of Camera Raw I appear to be up against the limits of my camera (that's another story... currently saving for an upgrade). There are solutions to this e.g. blurring sky to remove noise - which tends to turn up in large blocks of colour - but this seems to be a lot of work fiddling in Photoshop for no guarantee of a decent return i.e. time spent vs earnings. Which leads me to another possible solution... resizing the image to decrease the appearance of artifacting and noise. BUT obviously, you want to upload an image with as large a pixel area as possible, so it is available at the most sizes and you don't miss out on money because you resized the image and it was too small.
So what are the iStockphoto size minimums?
• XSmall, 300×400 = 0.12 MP minimum, 1″x 1.5″ @ 72dpi
• Small, 600×800 = 0.48 MP minimum, 2″x 3″ @ 72dpi
• Medium, 1200×1600 = 1.92 MP minimum, prints 4″ x 5″ @ 300dpi
• Large, 1920×2560 = 4.92 MP minimum, prints 6″ x 8″ @ 300dpi
• XLarge, 2800×4200 = 11.7 MP minimum, prints 9″ x 14″ @ 300dpi
• XXLarge, 3300×4900 = 16.2 MP minimum, prints 11″ x 16″ @ 300dpi
• XXXLarge, 3700×5600 = 20.72 MP minimum, prints 12″ x 18″ @ 300dpi
What this means is if you wanted to upload your image at the lowest boundary of the "medium" size it would need to be 1600 pixels x 1200 pixels (BTW: this is also the smallest size accepted by iStockphoto) - what this actually means is that the image pixel area must exceed 1,920,000 pixels (or 1.92 MP) and technically, you could submit an image that was 1 pixel wide x 1,920,000 pixels high! As long as the total image pixel area (i.e. the pixel width multiplied by the pixel height) is 1,920,000 pixels then it meets the minimum size requirements for the "medium" image size.