There are loads of articles comparing graphic file types examples can be found here and here. I don’t intend to add to this again. However, here are the graphic examples I was using in class. The resolution remains unchanged for each but the colour depth is decreased for the GIF (which has a max of 8bit colour).
PNG original (417KB)
Saved as a GIF (147KB) – Transparency added
Saved as a JPG (21KB) – Lots of compression