Giving a career talk in my old university, how prominently should I tell students my salary?

I agree with you. I feel the request to put your salary on a slide (on the first slide, no less!) is rather unexpected, and quite frankly does not speak highly about the professionalism of your contact. I would decline this, for multiple reasons:

• Your salary is nobody's business. Not sure what more there is to say about this.
• Students are, for the largest part, intelligent adults. Many would take this exactly for what it's meant to be - rather crude marketing and hype generation. This would detract from the message you actually want to transport, and would undermine your following talk.
• Focusing so much on how much you make is arguably not the best way to motivate young people for a specific career path anyway. If you talk about why you love your job (if you do) will encourage more people than a six-digits salary number. More importantly, it will probably encourage the people who will actually be happy doing your job, not the ones who would end up wealthy and miserable.

You can of course provide a salary range as proposed by other answers, but I would not emphasize this point much (and base it on third-party data, such as Glassdoor, not just your own experience).

I would think you can share the salary "Range", but I, like you, would not put it on the first slide.

ie after 3 years experience you could expect xxx to yyy as a zzzzzz.

This answers a second question that the OP had in the original post, now edited:

Sometimes students like to hear about a "real" problem and "how" it was solved - that process is usually interesting and can give them a "focus" of why they have to study xxxx.

As a statistician, I consider that an unrepresentative sample of one is unlikely to convey any useful information at all. You would mislead your audience if you presented them with such stuff.

Those of us with experience of the world know that amazingly high salaries are sometimes available to amazingly under-qualified people. So there is no point in presenting that stuff either.

What I wish someone had told me when I was aged about 20 is what kind of life goes with particular professions. So, in your case: what is it like to be a software engineer? Can you spend your whole career doing that? or, do you need to be on the lookout for something better/ less stressful / more stressful but better paid etc? Where do you hope to be in 10 years time? What is the career path? Is there a career path? If I do moderately well as a software engineer, what sort of place will I be living in?

Your actual salary is irrelevant.