year can change based on the item in question, but things usually enter the public domain at 50 or 75 years old. I read that Peter Pan will not enter the public domain as long as a certain hospital exists (which profits from the revenue Peter brings) and a few others. An artist can also forfeit her rights and put something into the public domain. 

SO, you are (probably) safe using something 75 years old, but verify anything newer.

@annfurnivall  I’ve used http://www.dreamstime.com/ in the past. Their selection is limited, but I have found images I can use there.