I remember that discussion. It was here and the relevant bit was
And I agree with @ralph_m - (a) if accreditation is required, it should be clearly visible, and (b) it would not adversely affect my impression of a site.
Probably. It would depend on the context and the overall layout. If all the images shared the same attribution, I might just state it once, perhaps in the footer.
Assuming I’ve decided to put the attribution under the image, then yes.
Put proper alt text in the alt attribute and not use title at all.