Until you print an image, you have no idea what the DPI is. For example, if your image is 1200 x 800px and you print a 4x6, then the image is 200 DPI -- if you print a 2x3, it's now 400 DPI. The calculation is as follows:
Width in pixels of the long side of the image, divided by inches of the long side of the print
So, a 4x6 is 200 DPI for this image because 1200 (long side of the image) divided by 6 (inches of long side of the print) = 200 DPI.
A key piece of information to understand is that a digital image has no DPI, only pixels. Even if you see that the EXIF data for a photo shows a DPI-value, that information is misleading. It is a fictitious number until you know what size print is ordered!
To be safe for printing an image so that it does not turn out pixilated, we recommend 120 DPI or higher for each image. As long as you do not see the Low DPI Warning message in red next to a given photo in an order with our lab partners through ShootProof, the DPI is sufficient for printing that size.
What to do if you see a "Low DPI" warning:
When you do see this warning notice, you need to replace the image with a larger version that meets the 120 DPI requirement for that print size (click the 'Replace Image' button next to the image in the order) to ensure the lab print does not turn out pixilated.
If you are cropping the image within ShootProof, note that the DPI will become lower than the example shown above because the lab would not be printing the full image size that you uploaded. Any cropping of the photo will be factored in by ShootProof to determine whether you see the Low DPI Warning - so again, if you do not see this warning, the image is okay to print.