This is only for the the brave of heart.
Do not mess about with your original images.
Make a copy of the images you want to reduce and put them in a folder on your desktop.
Open Terminal app (space + terminal)
Type cd and a space then drag the folder with your copied images onto the terminal to save mistyping the path to the folder.
it should look something like this,
sips -Z 640 *.jpeg
the -Z says keep the image aspect ratio
the 640 says reduce the longest length to 640 pixels (you may choose a different length- eg 1000)
the *.jpeg says do this for any file in this folder with the extension .jpeg (if your images are .jpg use .jpg instead)
The files in the folder have been reduced in size- in this case the 3Mb file IMG_0001.jpeg is now just 109Kb