Must I give the gift back?

You appear to be asking a question that would better be addressed to Ann Landers (now deceased) or successors or maybe a counselor or psychiatrist. If there was no written agreement, surely oral agreement controls (e.g. a contract). If there was no oral agreement (meeting of minds), then apparently the gift was officious. Actually, this sounds as if it is more nearly an engineering and moral issue. The engineering aspect is whether there is any practical means for your friend to get it back and whether you have a practical means to counter any action he/she might bring. The moral question is your sense of obligation (guilt) or none, and same for your ex-friend. On the legal side, when somebody whom you do not know, intentionally sends you something officiously (by mail or leaving it on your doorstep for example), you can generally keep it. The situation you describe is nuanced, based your knowing each other, and the allegation that each of you understood impliedly conflicting terms (e.g. no meeting of the minds). Assuming the value of the article supported legal action, in theory a factfinder would decide the facts based on evidence, and a court would apply the law based on the determination by factfinder.

A gift is a gift, whether it's worth a few cents or a million dollars. Based on the information you provided, you don't need to give the picture back.

NO. A gift need not be returned.

No