The way cards are recognized by the MacOS is not what I'd call consistent. I have a 17" MBP with an ExpressCard/34 slot, and I have both a CF and an SD reader for that port. Mounting a CF card does not cause Photos to open, and the card shows up on the desktop as an orange 'removable drive' icon (just like my LaCie Rugged 1 TB backup drives). Mounting the same CF card using a FW800 CF reader launches Photos and it mounts as a white 'fixed drive' icon (like a typical thumb/flash drive). Mounting an SD card through the ExpressCard/34 port (different reader than for CF cards) opens Photos and it mounts as a white drive icon. So, there are flags set on cards for drive type, but how you connect the card also matters.
Regardless, there are a couple of ways to stop it. Both involve changing a setting in the Image Capture application (which you may not even have known existed, but it should be in your Applications folder). That's not intuituve by any means, but the setting in Image Capture trumps Photos, iPhoto, Aperture, etc. Historically, that was the only way to stop iPhoto from autolaunching – there was no setting to control the behavior in iPhoto preferences, only in the Image Capture preferences. However, Image Capture no longer has a preferences dialog at all.
The first way is to insert a card, quit Photos (or not, probably doesn't matter), then open Image Capture and you should see your card. Click on the little expansion triangle button in the lower left corner (not the eject button next to the card), then set the popup menu to No Application as in the screenshots below. If you use both CF and SD cards, you'll have to do this for each card type.
Alternatively, if you're comfortable in Terminal, the following command should take care of it in one step for all card types, and you don't need a card mounted to do it.
defaults -currentHost write com.apple.ImageCapture disableHotPlug -bool YES
Should you ever want to revert to the default behavior (why? no idea, but just in case), just repeat the above Terminal command, changing the YES to NO.
Hope that helps...