Here are a couple of websites that allow you and your kids to add text to images, generating a new image that contains the text, that you can then download to your computer.
At imagegenerator.net, add text to a speech bubble coming from Dumbledore, Napoleon, ex-President Bush, and others.
At hetemeel.com, you can generate an image with your own text superimposed on a picture of Einstein writing on a blackboard, an Uncle Sam poster, a dictionary entry and more. (The dictionary one allows you to upload an image as in my Book Chook entry far below.)
If you're looking for online places that encourage kids to do some quick writing and generates a savable image, give these two a try. The image could be sent to friends or family as an email attachment, or printed out. As always, parental guidance is recommended with any of these unmoderated sites.
And if you need to generate a quick image to advertise a cause dear to your heart, or a blog post, or an event in your classroom, you might find what you need there too.
(Check out two other poster-style generators in Cow Appreciation Day and Keep Calm and Carry On.)