If you've upgraded to recent versions of Mac OSX and inserted a memory card or attached a photographic device, you will have run into the new Photos app. It's a replacement for the old iPhotos app that also incorporates aspects of Aperture and integrates the management of photos more tightly into the operating system in a more IOS-kind of way.
The Photos app is at the heart of Apple's new approach to working with images, whether you like it or not. When you attach a camera to your computer or insert a memory card into a card reader it will automatically as you if you want to use the Photos app.
That's great if you do want to use the Photos app. But what if you don't? That popup every time gets annoying. And, I'm sorry to say, I'm not much of a fan of the Photos app myself.
Thankfully, there's are ways to turn it off.
It involves using terminal. So fire up a Terminal window and copy and paste this line at the prompt:
defaults -currentHost write com.apple.ImageCapture disableHotPlug -bool YES
Then press Return. You won't get any feedback--it'll just start with a new, blank prompt on the next line--but that should stop it completely.
If you decide later that you want to revert back to the default behavior, simply use this to switch back:
defaults -currentHost write com.apple.ImageCapture disableHotPlug -bool NO
Per Card or Device or Customize Action
The global option is an all-or-nothing switch. But you can also fine-tune it more by specifying on a per-card or per-device basis whether simply to disable the automatic Photos launch or launch a different app or import automatically. It' uses the new Image Capture app, which incorporates the same kind of functionality as the old Cameras app.
The Image Capture app is now part of the operating system and is installed automatically.
What you want to do is open Image Capture (Applications > Image Capture). When you insert a memory card or connect a camera, it will show up under the devices section at the top left.
With the device you want to work with selected, click on the small menu icon in the bottom left.
That will expand a section that displays what action is set to happen automatically when that device is connected. By default, you should have it showing the Photos app.
You can change this here to several options, including "no application," in which case nothing will happen. You can also set it to import the images automatically do a specific place on your hard drive or open something like Lightroom.
Once you change that, you're all done for that device. There's no save settings button. Just close Image Capture, and the next time you insert that card or connect that device your chosen action should happen.
The settings apply only on a per-device basis, unfortunately--there's no one-click way to set a particular action for all devices.