Wrong! Some of Great-grandpa's other descendants may be looking for information about him. They, too, would love to see a picture of him when he was a little boy — even better, they'd love to have their own electronic copy of that picture!
As you develop more online contact with other genealogists, you may find a lot of people who are interested in exchanging information. Some may be interested in your research findings because you share common ancestors, and others may be interested because they're researching in the same geographical area where your ancestors were from. Aren't these the same reasons that you're interested in seeing other researchers' stuff? Sharing your information is likely to encourage others to share theirs with you. Exchanging information with others may enable you to fill in some gaps in your own research efforts. Even if the research findings you receive from others doesn't directly answer questions about your ancestors, they may give you clues about where to find more information to fill in the blanks.
Also, just because you haven't traced your genealogy back to the Middle Ages doesn't mean that your information isn't valuable. Although you should not share information on living persons without their explicit permission, feel free to share any facts you know about deceased ancestors. Just as you may not know your genealogy any further than your great-grandfather, someone else may be in the same boat — and with the same person! Meeting with that fellow researcher can lead to a mutual research relationship that can produce a lot more information in a shorter amount of time.