Slider images different widths

I’ve successfully installed OTW Portfolio and created some categories and projects. I just have one issue. You can see problem here: http://sturmanarchitects.com/dev/projects/cape-on-the-rolling-hill/

I think the problem I’m experiencing is because the width of the slider images varies (some are narrower than others). When I click on individual slider images the main image below rotates as it should, but eventually things get off sync as I go along.

To see the effect try clicking one by one on each slider image. Soon you’ll see that the slider images are scrolling way too often compared to the main image below.

I could probably fix the problem by making sure all uploaded images are the same size, but I really can’t do that since my client wants the photos exactly as photographed.

Is there any way to force the images in the top slider to be the same width? I don’t want the images distorted, but I wouldn’t mind if the narrower images had white padding on each side so that the <li> widths were consistent. I tried hard-coding the width of the <li> tags in otw-portfolio-post.php, but that had no effect. How can I set the width of the <li> tags in the slider so that they’re all the same (regardless of the actual image width)?

Any help would be greatly appreciated! Thanks.
https://wordpress.org/plugins/otw-portfolio-light/