If you're unsure which device will be used to view an image, a 16:9 aspect ratio is often considered a safe and versatile choice. This aspect ratio is widely used across various devices, including desktop monitors, laptops, tablets, and smartphones, making it suitable for most viewing scenarios.
However, if you're aiming for even greater versatility and compatibility with a wider range of devices, consider using a 1:1 square aspect ratio or a 4:3 aspect ratio, as these formats are also commonly supported across many platforms.
Ultimately, the best choice may depend on the specific context and content of the image, but 16:9 is a strong candidate for general-purpose use.