I have used SIG (the free version for about 6-7 years and the Pro version for the past 4-5 years). I have 'always' had to add CSS override coding to correct thumbnail borders. I use Joomla on all of my client websites, all are updated to the most recent version, I use the Gantry platform which is the most recent version. It has been a perplexing problem.

webbering.com/images/troubleshooting/sig-thumb-borfder-problem.jpg

Here is the code for a specific thumbnail. It forces a "style="width:200px;height:160px;" " for the <A> tag. However, it also forces a "style="width:200px;height:160px;" in the <IMG> tag. If I view with developer tools, it is calculating the <IMG> tag as "img.sigProImg 172x160". However, in the <A> tag, it calculates the size as "a.sigProLink.fresco 200x172". With a 4 pixel border, I think you can see the problem. The calculation of the <IMG> tag should be "172x144" as the <A> tag contains the "200x160" image background to match up with the 200x160 settings in the component.

<span class="sigProLinkWrapper">
<a href="/images/gallery/2019-04-12-VOCAL-NCVRW-Vigil/2019-04-12-VOCAL-NCVRW-Vigil-001.jpg" class="sigProLink fresco" style="width:200px;height:160px;" rel="fresco[galleryd0f096cea0]" title="" data-fresco-caption="" target="_blank" data-thumb="/cache/jw_sigpro/jwsigpro_cache_d0f096cea0_2019-04-12-vocal-ncvrw-vigil-001.jpg" data-fresco-group="d0f096cea0">
<img class="sigProImg" src="/plugins/content/jw_sigpro/jw_sigpro/includes/images/transparent.gif" alt="Click to enlarge image 2019-04-12-VOCAL-NCVRW-Vigil-001.jpg" title="Click to enlarge image 2019-04-12-VOCAL-NCVRW-Vigil-001.jpg" style="width:200px;height:160px;background-image:url('/cache/jw_sigpro/jwsigpro_cache_d0f096cea0_2019-04-12-vocal-ncvrw-vigil-001.jpg');"></a>
</span>

I have tried enabling "Smart thumbnail resize based on above dimensions" but it had no effect.

I believe the culprit is the "height:auto !important" setting in the CSS.

Here is the link:
vocalonline.org/news/vocal-news/291-2019-vocal-vigil-honoring-our-past-creating-hope-for-the-future

If you have any suggestion, I would appreciate some feedback.