I don't know why people are down-voting your post. It is indeed inappropriate to ask people you're in touch with mainly for language exchange for additional pictures of themselves. What purpose does it serve? Somehow, I get the feeling straight men talking to other men for language exchange don't request photos, and nor do straight women talking to other women. This shows the behavior is inappropriate.

I wouldn't dream of asking the women I speak to online for photos. If they want to share photos - usually to show me a place they visited or illustrate a point - they are the ones who initiate it.

To be asked out of the blue for your picture by strangers is very weird.

I think asking for a picture or using some expressions with strong sexual connotation is too much. But not all requests are necessarily harassment. I think there is also a cultural component. Where I live, asking some personal information (age, marital status, children, where do you live, about your job, studies, etc.) is the polite and usual way of showing interest to who you are talking with. Other cultures are more sensitive to this kind of requests (and vice versa), but I think you can state these limits during the conversation.