The resolution of an image isn't determined by its file format but by its pixel dimensions and DPI. So, any file — including a JPG — can be a hi-res image provided it has large pixel dimensions and a high DPI (usually 300 or more). It's important to note, however, that JPG is a lossy format. This means some image data and quality is lost each time the image is saved, which could affect the resolution over time. You can learn more about different file types here.