Only the file size matters for loading speed, not the image type.
For simple graphics, not pictures, gif is usually the smallest file size. But gif is NOT suitable for pictures, it distorts them, making them blotchy looking.
For pictures, jpg is usually the smallest file size. But jpg is NOT suitable for simple graphics, especially those with large areas of single colours. It distorts them, making them blotching looking.
png can be used for both simple graphics and pictures, it doesn't distort either of them and has more advanced transparency than gif. But png files tend to be bigger than the corresponding gif of jpg.
So chose the files type that suits your needs.
For small thumbnails, especially those that are a mere 40x40, I'd say use jpg for pictures, unless its simple graphics then use gif. For artistic type transparency effects in an clever page design, png is your best choice, for simple graphics, use gif.
