To set a background image for a table row in HTML that fits perfectly without repeating or leaving gaps, using CSS is the most effective approach. Specifying the image dimensions directly in HTML with width and height attributes can lead to issues when the window is resized, as the image may not scale properly with the row. Instead, using CSS allows for more flexibility. Setting the background image in CSS with properties like `background-size: cover;` ensures the image covers the entire row without repeating, adapting to the row's dimensions. If the image appears too small or leaves space, adjusting the `background-size` to `contain` or using specific pixel values can help achieve the desired fit. The key is to ensure the CSS properties are correctly applied to the table row for optimal results.