This image has been generated by a batch conversion – the original image is very big and has a transparent background.
In IrfanView, you can modify the output color of the finished image. To do that, you need to go into the main application, not the advanced settings in the batch concersion window.
Click on Options -> Properties/Settings.
Select the Viewing entry here, and go to Window options.
Click the Choose button, to set the Main window color. This will be the color which your batch converted images will have as a background, after they are converted.
A bit counterintuitive. Also a good example for the limitations of ChatGPT: It was telling me to “Look for an option that says something like “Set background color” or “Change transparent color to:”. Check the box beside it and then select white as the color.”
When a human would say something like that (“something like”), we could infer that they actually don’t 100 % know. Possibly with ChatGPT that’s shown in a similar way.