[Imp]

Pat reload your image in to whatever program you use.
Select save as - not save.
You should get a selection of file formats
Select JPG or JPEG
Save.

File size is about 10 times smaller
If your program gives options for the JPG just accept the default.

NOTE You can trim the quality back to 80% with little noticeable difference in quality if needed to reduce file size further. Ideal for all but high quality stuff you want to print or look at on a big screen.