You could say any image is objectifying, it is ludicrous. Eventually we need to all stop being offended about everything. Use of the Lena image is not keeping anyone out of the CS field. There is great value in students learning how their research fits into the broader context of the field and learning that in high school is an excellent way to help prepare students to do more advanced work. In this particular field the bulk of the research over the past 40 years was at leasted tested against the Lena image so it is a great image to use.
There is nothing actually offensive about the image. If you have to go beyond what you are looking at in order to get offended, just stop. The problem is you.