What image viewing software works on MacBook Pro with macOS 10.11.6?

Max OS X El Capitan shipped with Apple’s Photos application which was a direct replacement for iPhoto. The app is installed with the OS and cannot be downloaded and installed separately. Updates would have been installed with updates to the OS.

OS X El Capitan - Technical Specifications - Apple Support

User @Matti Haveri mentioned Graphic Converter which is a great editor, if you can find the download for such an old OS. Google it and check the publisher’s website.

PETERV-6 (Author)

For different idea on this, to use an 'image editor' such

as ToyViewer. Not from mac app store ~ that's too new..

This was initially Made for Mac OS X; older still may be

found in working order in URLs from those days.

My older early 2009 iMac 24-inch runs El Capitan and

this uses older version ToyViewer 5 from its maker site.

One from the Snow Leopard era (10.6.) works in 10.11.6

While one for Leopard 10.5 a Universal version & works.

ToyViewer for Mac OS X maker's first site was Japanese.

//duckduckgo.com/?q=T.+Ogihara+ToyViewer+for+Mac&t=ftsa&ia=web

As I also have a Late 2005 Mac mini 1.5GHz (PPC) this

runs Leopard 10.5.8; and same one works, in El Capitan.

Each edit of image in ToyViewer creates full new image.

And may be useful to edit single-frame video images.

As an image editor this has many features often found

among higher end retail products; back in the day. As

such, superficial glance? May miss most its greatness.

I've also Graphic Converter among vintage macOS s/w.

I used GraphicConverter 11.6 in El Capitan but officially it needs GraphicConverter 10.

https://www.lemkesoft.de/en/products/graphicconverter/download/download-old-versions

Some old GraphicConverter version or of course the bundled Preview.app or the old iPhoto.app.

.heic support was added in macOS 10.12 but it can not display the new iOS 18 .heic format anyways.