Yes, if you zip a file, you are compressing it. You are making it smaller for emailing to someone. So far, what you think is right.

But zipping does absolutely nothing – at least nothing useful – if what you’re sending is a JPEG image or an MP3 file. Because those are already compressed. If they could be compressed further, they would’ve been.

What you’re doing by zipping an image or a MP3 audio is making it tougher for the recipient to open. I just had a thing where I had to unpick why an audio file wouldn’t play on an iPhone: it was because it was sent zipped. All zipping did was make the audio unplayable until I’d unzipped it and resent.

So you are always better off sending JPEG images and MP3 files unzipped.

But.

There is one specific situation when you should zip anyway.

It’s when you’re sending many files to someone at once. Zipping produces one file and it does so regardless of how many things you’re trying to send. So, for instance, I just delivered 40 images to a client and did so by sending them one zip file containing the lot.

They get one file and it has everything.

Now, what I actually sent them was a Dropbox link to the zip file, I didn’t try to send 100Mb of zip to them over email. But the zipping part was the same.

So.

Zip to compress files like word processor documents and zip to gather up lots of files into one.

But don’t zip a JPEG or an MP3 because that’s just a chore.