Not actually stolen from Google, legally downloaded flashcards… But, if there's no involvement, that's almost as bad!
Why is it wrong to steal images?
First a misconception most users are under: no one has given Google (or any other search engine) permission to grab all their images off their websites. It is therefore wrong for Google to steal images in the first place — and that doesn't make it right for you to steal the same images from Google. Neither does the fact that "everyone else does".
As an educator, I think that you have a moral responsibility: you cannot condone and must condemn theft, and therefore can neither steal other people's images and use them, for example, on a blog, nor allow your student's to do that.
Images or content — text, that is.
Why bother creating your own?
But there's more to it than the moral considerations, which I imagine are not going to convince many people nowadays.
Teach your learners not to copy and paste, but to copy and paste; select judiciously, cut ruthlessly and quote correctly…
For example, with text, in a webquest you have asked your learners to find the answers to certain questions, and then to "publish" them in some way — in a Word document, as a PowerPoint presentation, on a blog. If they merely "copy and paste", as many will do, they are very unlikely to be doing very much manipulation of the language; they are not getting to grips with it, getting "under the bonnet" and getting their hands dirty, tinkering with it and reformulating it in any way. Merely copying and pasting it isn't going to do a lot for their language learning.
Teach your learners not to copy and paste, but to copy and paste; select judiciously, cut ruthlessly and quote correctly… And provide the language for that ("(….), according to Yahoo News"; "says a report on CNN" [+hyperlink]; etc)
If it's images, it's far better for them to create their own, than handle stolen property from Google. Why?
What you want, for learners to be truly engaged in their learning, is for them to be creative, to be imaginative. Google Images is not creative.
You want affective involvement in their learning — you want them to care. When they do, they learn more.
What you want is for them to be proud of what they've created. There isn't a lot of pride to be had in handling stolen goods.
How do they create their own images?
Images can come from digital cameras, from mobile phones, from hand-drawn art work (which is then either photographed or scanned), or be created in a simple image editing program like Paint.
Is it worth the effort?
Yes. Why? Because of the pride to be had in the creation…