Hey there. It is true that taking a screenshot is probably the safest way to access a photo instead of downloading it. With the case of youtube videos, there was a study conducted by Georgetown University that showed how hackers can hack your smartphone while you watch videos on your pc. This type of hacking uses voice recognition. The hackers create videos that will elicit a certain vocal reaction from you such as cute videos that will make you say 'aaww.' From that factor your smartphone is hacked. With Facebook most hacking happens when you open suspicious links.