Picnik for a while. I registered there to use the free image editing service a year or so ago. But recently I decided to avail myself of more Picnik features by subscribing to their Premium content. It costs me about $25/year. So far, it's money well-spent.
What I like:
*Before I signed up for Premium, I liked that Picnik very generously has lots of free content. Premium has given me access to (for example) more stamps (clipart), more editing tools etc and the ability to upload up to 100 images at once. I just use my Shift key for the latter feature.
*Picnik is online, so I can use it from anywhere in the world I happen to be. If I'm away from my home image editing software, all I need is my password, and I'm able to use all the Picnik features, both free and Premium.
*Picnik users seem to be a happy, sharing bunch. There are zillions of tutorials online telling us how to do things at Picnik. And Picnik itself has lots of helpful articles. They also have a blog.
*Picnik is easy enough for me to use, and use quickly. Yes there are lots of tutorials, but to do most jobs, I didn't need them. I often use Picnik to add text to an image, edit an image with a special effect, make a collage of images, or add a border to an image. And I love that I can create an image from scratch, just by choosing a blank collage template as my background, and building from there with stamps, text and a border, the way I did above. I can probably do those things in Photoshop, but it takes me so much less time at Picnik. It works all the time and it does most of what I want.
I love playing with words and images. Picnik has contributed to that love by providing a simple, intuitive interface. I think you and your kids would enjoy playing there. I've used Picnik recently to help create images in Let's Celebrate Talk Like a Pirate Day, and Link Reading and Craft with Bookmarks.
If you're interested in discovering more ideas on this topic, try my recent article, Book Chook Favourites - Playing with Words and Pictures, or browse articles under Creating in the right sidebar here at www.thebookchook.com.
UPDATE: Picnik has gone. Long live Ribbet!