The screenshot is from my desktop with wide enough screen on Lemmy web (programming.dev).
The issue is one of scaling.
When I open the image without being resized into the website layout, it has the following visual pattern:

When I zoom out to 50% it looks (almost?) fine

Did you scale the source with ffmpeg? Do you have a visual pattern in your console background? The simplest solution would be to have a solid color as background. The second best to render a small enough size that it does not get resized in the browser.
At 1920x1038, it's very big right now. I'm surprised the font is big enough to be readable. I assume you scaled it up or have a high dpi display resulting in this.
