The Costco here in my home town only excepts Jpeg files.
You will need to call your local Costco to fine out the type files they will except or if you are doing this online, it is a simple matter of just calling the photo department to find this information out.
Once I have all my editing completed, I save the file as a Jpeg and I save it to my compact flash card to take to the store. If I am doing this online its just a matter of entering the Costco Internet site and following their direction in submitting photographs.
There is no need to save your file in 16bit ... 8 bit is all that is needed.
If you have color calibrated monitor you could download the profiles from Dry Creek and softproof your ediiting.
http://www.drycreekphoto.com/icc/
If not don't worry about it.
Once you are happy with you editing ... it will then be time to increase your print to the output size of 20"x30".
Sharpen your print to your llkes after you resizing to the 20x30 size.
Oh and by way make sure you tell the lab, NO COLOR CORRECTION
Hope your print turns out great ... Costco will make sure you are happy with the final outcome if you have done your part correctly.
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We suffer from so little individualism, we are so quick to replicate rather than innovate.