There is a lot of cute clip art out there for classroom use for the younger grades, but it's hard to find artwork to use for older kids. I complained to my artist daughter loudly and often enough that she started producing more mature clip art for classroom and product use. Using artwork on worksheets and Power Points meant for older kids is tricky - if it's too cutesy they won't like it, but boring doesn't add anything. You also don't want to overdo it. A few well placed pieces are plenty.
I tend to buy clip art, use it once and forget about it, but I keep coming back to these images again and again.
This is her most recent freebie, which you can download here, from her Image Boutique store on TeachersPayTeachers. She has several other freebies which you might want to use for a test run in your classroom.