It is not rude to not respond to a Snapchat selfie with no caption or context, as it is understandable that sometimes people may not have anything to say or may not want to engage in a conversation. However, if you do choose to respond, it's important to be mindful of the other person's feelings and avoid coming across as rude or dismissive.
Instead of ignoring the selfie, you could try responding with a simple acknowledgement, such as Hey, thanks for sharing! or Cool, looks like you're having a great day! This shows that you've taken the time to notice their selfie and appreciate their effort to share it with you.
Alternatively, if you don't feel like responding to the selfie, you could simply let the conversation fade away. It's important to remember that people have different communication styles and preferences, and it's okay if you don't always feel like engaging in a conversation.
Ultimately, the key is to be respectful and considerate of the other person's feelings, whether you choose to respond or not. Avoid ignoring their message or responding in a way that comes across as dismissive or rude, as this can create misunderstandings and hurt feelings.