Usually this occurs when the media is actually 1440 x 1080 using non-square pixels. PP knows how to 'unsquish' the clip so it looks normal, but most photo views don't.
Export the frame as 1920x1080 square pixel with AME instead of the snapshot button.
Below is a step by step guide with pictures so keep scrolling. You'll have to figure out how to apply these steps with other software.
The easiet solution is offered by Ann Bens. Here is a screen shot of how I used the AME. Make sure you UNCHECK export sequence and format to jpg or tiff.
Its quicker to use the export frame button on the monitor but if your going to use the end product as a still then you have to spend the same or more time fixing it; the AME is the best solution. Here is how to fix a jpg using photoshop.
STEP 1 - you'll see this window upon opening the file exported from PPro in PS.
STEP 2 - You'll see a widescreen picture but ...
once you uncheck "pixel aspect ratio correction" you see it looks squished
STEP 3 you'll want to resize it in the following sequence
Change the pixels from 72 to 300
B. Change the width to either 10 or 5
C. UNCHECK "constrain proportions" and change the dimension to 1920 Wide and 1080 Height
Now save the file and its ready to use in most other software and for use with online services like Costco's photo books and printers.