Do not worry, the template does not overwrite images, in demo some images were gray from the beginning and for others we used only simple CSS3 rule to add grayscale filter to change the color of all images to black and white (100% gray). Check this part of CSS code:

.sppb-addon-articles .sppb-addon-article a img {
  -webkit-filter: grayscale(100%);
  filter: grayscale(100%);
  -webkit-transition: .4s;
  transition: .4s;
}

Legend:

• 0% (0) is default and represents the original image (color in most cases).
• 100% will make the image completely gray (used for black and white images).

Of course using custom.css file you can override value for linked images, for example:

.sppb-addon-articles .sppb-addon-article a img, article.item.item-page .entry-image img, 
#sp-left .sp-module .latestnews > div .article-list-img img, 
#sp-right .sp-module .latestnews > div .article-list-img img {
  -webkit-filter: grayscale(0);
  filter: grayscale(0);
}