Using an online service, like Tinypng or Optimizilla, will allow you to compress images. Reducing file size without losing image quality is the key here, but both of these services are up to the task. Optimizilla gives you realtime feedback on quality and file size, while Tinypng will automatically give you the best result without any image quality loss.
Check out the image below for a side-by-side comparison of an image that has and has not been optimized. You probably won’t be able to see the difference, but your mobile visitors will thank you.