Hi there =)
I think it’s because you are using a transparent image.
It happens because the html structure displays the block image twice depending of the portfolio mode (one in a tag, and the other one as a background through CSS).
You have 2 ways to fix this:
Option #1 – Add a white background to your image (using a image editor). – Recommended
Option #2 – Add some CSS to remove the background image. You can add the following CSS code to Appearance > Customize > Additional CSS:
.elementor-14 .elementor-element.elementor-element-f6be058 .elpt-portfolio-content .portfolio-item {background: none !important;}
However, some portfolio modes will not work as expected. That’s why we recommend the first option.
Please let us know if it worked for you.
PS: It does not have impact on page load performance/speed, because the 2 images are the same file.