All phones, depending on their megapixel abilities create different resolution photos. The number of pixels in a mobile photo corresponds with the number of photosites/pixels on the camera’s sensor. We have created a table with the most common phone cameras and the corresponding ideal print sizes here: how big you can print.
The great thing about our intuitive software is that our system will alert you if the image is too low resolution. In your project; you will see the following red alert in the bottom left corner of the uploaded image….
These two images both look okay on screen, however, notice the red alert icon in the bottom left of the 2nd image… 
This is a low-resolution warning, and means your photo does not meet the minimum resolution requirements. 
In this instance, you could either try a higher res copy of the photo or choose to change the size of your print to a smaller size.
How do I check the resolution of my photo? Windows vs Mac OSX
To check a photo’s resolution on a Windows PC, select the file you want to use. Right-click on the image and then select Properties.
A window will appear with the image’s details. Go to the Details tab to see the image’s dimensions and resolution.
Mac OSX
Checking a photo’s resolution on a Mac is similar to a PC. Select the image you want to use and Right-click on it. Click on Get Info.
The image information window will open. You can find the image dimension and resolution under the More Info tab.
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