If you want to know what image file type has already been compressed then take a look at this table.
JPG / JPEG
Lossy or Lossless
As you can see from the table above, most image file formats you may have heard of are already compressed. Compression can be one of two formats; either lossless or lossy. A lossless compression method won’t lose any quality when you apply it. So for example no matter what format you store an image in, it will look exactly the same if lossless compression is used. On the other hand, a lossy compression method means you will loose some quality. This isn’t a problem for most images as most people won’t be able to see the difference between a compressed and uncompressed version. For example, this is a lossy compressed JPEG: It looks quite high quality but it only needs 173KB of space to store it. The same file came out as 1.8MB when using a lossless format like PNG - that’s about ten times as big!
Because images in a raw file format can take up a huge amount of space on your computer or digital camera you don’t see them that often. The TIFF file format does offer an uncompressed format which can be used. Other than that you may have seen the Bitmap (BMP) file format being used to hold uncompressed images. You’ll notice them by their massive file sizes!
The majority of modern image file formats have been compressed already. When choosing images for your projects, you might just want to decide whether you should use a lossless image or a lossy image. As shown in the example above, the compressed JPEG image is of a sufficient quality for most projects. The only real disadvantage with them is they don’t support transparency for which you would need to use a PNG file. Looking for images for your next project? Check out the 14 best places to find quality images for websites article.