How to get rid of white ugly pixels on thumbnails.

Hey, does anyone know how to change the color of, or delete, the row/column
that is being outtputted when I make a thumbnail?

I'm Using PrestaShop e-ciommerce software that automatically generates
thumbnails of all my product photos.  But I get either a row or a column
(depending on the dimension of the thumbnail) of white pixels that are
leftover either on the bottom of the thumbnail, or on the right side of the
thumbnail, respectively.

My website is mostly dark colored, so these extra white pixels clash with my
design.  Does anyone know how to delete them or change their color (via
PHP?)?

Thanks!
-Joe Pea