I would never have a problem with it.If something I did aids another it makes me more happy than a worry about legalities. If you ever sold a work and made millions from it I would think sending the person a percentage would be a nice gesture though.LOL Maybe there are some who do not like any of the images they create being used. I can see that happening when it concerns people portraits.

"The happiness of a man in this life does not consist in the absence but in the mastery of his passions."