I understand (more or less) that when printing with an inkjet printer it is desirable to resize an image so that it matches the printer's native resolution.
Hi,
That's basically correct, although not limited to inkjet printing. The reasons that it is preferred are that you can use better quality resampling, and you can do output sharpening
at the final output size.
What is the best way to size photos to be printed using online services like Shutterfly, which I don't think use inkjet printers? Do you just leave the resolution as is, or are there situations where it would be better to upres?
I have no idea what equipment they use for printing, but it's that equipment (and the print settings they use) that would dictate the best output size to deliver to them. So I would ask (or search on their website) for the the required output equipment specifications that you'd like to match. Their website doesn't help much, since
they state:
What about DPI (Dots Per Inch), etc?
Shutterfly bases a good "resolution" recommendation on the final resolution output of an image and therefore can't give a good recommendation on PPI (Pixels per Inch), LPI (Lines per Inch), or DPI (Dots per Inch). Please look at the final output resolution after the image is created to determine if it will print well on Shutterfly based on the above recommended resolutions.
Those recommendations for file size in megapixels suggest pretty low (100-200 PPI) resolution input, but that's probably to keep a reasonable upload bandwidth performance. There's a pretty good chance that they use equipment with a 300 PPI native resolution, but it could be different/higher for the larger sized output.
Cheers,
Bart