The reason why cats look at butterfly pictures may be because they are interested in or want to catch these small animals. Butterfly drawings are usually images or designs of butterflies rather than actual butterflies. Cats' reactions to butterfly pictures may be due to their natural interest in moving things, or because they associate real butterflies when they see butterfly pictures. Cats may also look at butterfly pictures because they are playing. Many cats love to play and will use various methods to catch or chase things. Butterfly pictures may be one of the objects they play with.
To sum up, cats may look at butterfly pictures because they are interested, want to catch butterflies or are playing. These reactions are normal cat behaviors.