Compressing multiple pictures on Mac to save storage space

With Photos, you have the option of using iCloud and Optimizing the storage. Doing that, you still have full use of the pictures. You can scan through them all and, after loading from iCloud, they can be edited and printed. If you use Optimize, though, you can't back them up easily.

If you are talking about files floating around on Finder, then they certainly can be compressed, but the thing is for pictures, JPG and HEIC are already compressed. There is no advantage in trying to compress them further.

Get an external SSD and copy your libraries, Photos, Music, etc., to it.

NOTE: be sure to test the libraries on the external SSD before deleting the originals from the boot drive.

An excellent source of external drives is OWC (MacSales.com), particularly this one: the Envoy Pro Mini. It's easy to carry along with the laptop.

If budget is a concern consider this setup;

I have several of the 1 TB size with my Mac Mini M4.

OWC is considered to the the premier 3rd party hardware supplier for Macs.