I don't think I really understand this since most of things seemed to be a bit silly, but I could see how teens would want to ask questions about how these. My big concern would be to make sure that virus protection is running strong before sending people out to look for image generators, but that's just me.
I tried a site http://www.jellymuffin.com/ but it was terrible. Mashable worked a lot better for me and had more mature looking options available. This wasn't an online generator itself, but recommended different sites for what it was you're trying to do.
I chose the option to create a movie poster. It didn't turn out very well since I didn't have the right size image, but I get the point. http://bighugelabs.com/