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google is you friend for this one

this was the first results i got:
https://forums.macrumors.com/archive/index.php/t-323815.html

the key solution being: "to turn this off for all cameras, open Image Capture, and go into it's preferences. there's a drop down box for what you want done when a camera is connected."
(Image Capture will be within your Applications folder, not far from iPhoto probably)

if you'd still like you digital camera to auto-load iPhoto when connected, then this used to be a good solution - not sure if it still works though:
http://www.37signals.com/svn/posts/873-tip-prevent-iphoto-from-opening-when-you-plug-in-your-iphone
 
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