Increasing pixel size?

I have photos I wish to sell online via istockphoto.com. I have just discovered that they require photos to be sized 1600 X 1200 pixels whereas mine all seem fall just under this requirement (i.e., 1575 X 1175. Is there any way to fix this and secondly, how do you know pixel size in the first place: simply by print... show more I have photos I wish to sell online via istockphoto.com. I have just discovered that they require photos to be sized 1600 X 1200 pixels whereas mine all seem fall just under this requirement (i.e., 1575 X 1175. Is there any way to fix this and secondly, how do you know pixel size in the first place: simply by print size or somethong else?

Thanks in advance for any help anyone can provide.
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