When a girl sends a picture of another girl to a guy, it can have different meanings depending on the context and the relationship between them. In some cases, it could be a form of flirting or a way to show interest. For example, if the girl sending the picture is attracted to the guy, she might send a picture of another girl to gauge his reaction or to see if he shares her interests. Alternatively, the girl might send the picture simply as a friendly gesture, to share something she finds interesting or attractive, or to start a conversation.
On the other hand, if the girl sending the picture has a romantic or sexual interest in the guy, the picture could be a form of sexual or romantic innuendo. In this case, the picture might be intended to suggest that the girl finds the other girl attractive, and by extension, she might be hoping to suggest that the guy finds her attractive as well. However, it's important to note that this interpretation is not always the case, and it's important to consider the context and the relationship between the two people before making assumptions.