-We can use any image on the internet (No need to care about it is a stock image or copyrighted image)
A copyright violation might get you a copyright claim. But that's a lot more likely if you do it on your (business) website than on a public forum.
-For copyright image, the source should be given
You should always give the source.
-For general images, no need for source (because it take a lot of time mentioning the source if you post a lot of images in post as i do)
I don't really know what you mean with "general images", almost anything has copyright. Including this text I just typed!
Posting a lot of images is no valid reason to omit the source. If anything, it should be an extra reason to add sources.
-If any other user posted that image / Meme before on bitcointalk, i can post it too. (this is i am saying because a lot of images are posted in WO thread and we can't check if the image or chart is posted before or not)
You don't have to check all 50 million posts before posting anything. But if you copy it from another post, you should just use "quote" instead.