Imported images being stretched to fill a sequence frame width

Using a compilation of image and video media

I prefer to resize my images so the height is the same as the sequence frame height. If you resize your images to the same height as your sequence frame height and are happy to keep it at its native aspect ratio, rather than crop it to the sequence aspect ratio prior to importing it then you will have to accept the black (or base colour) matting on each side of it that makes up the difference in image width to frame width. Premiere Pro has options available to fill the frame if that is desired. If by default it is doing that and you don't wish for that, because it stretches an image from its default width to the sequence frame width distorting the image, then the first thing you should check is the "Pixel Aspect Ratio" for that image. It may have to be set to "Conform to Square Pixels (1.0). For me, this setting resulted in the image remaining at its imported width. Its height was already fixed to the sequence height before it was imported into Premiere Pro.

In CC 2015.3 this option is found by right-clicking on the image asset in the Project panel then > Modify > Interpret Footage. Look for the "Pixel Aspect Ratio" section.

Hopefully that is all you will need to do.