Not all images I used for these Acrosstics are probably public domain, and some might even have had marks I cut off. But mostly, if I cut off the mark, I mention the artist in the text. Also, I adhere to the fair use conditions of not wanting to pass on the image as my own, using it instead like raw material for a totally new purpose, a backdrop to complete my particular niche of art.
So, I ask forgiveness from those who I may have wronged, and thank those who allowed me to reast on their shoulders. Leat's just make this world a better one...
And remember the founder of the Python programming language, who based it on the mantra: "It is better to ask forgiveness than to ask for permission." It does save a lot of time, because asking beforehand often gets you no answer or is totally undoable because you find no address.