Picnik is one of the better browser based photo editing sites that you will find. Browser based means that Picnik simply runs in your web browser and does not have to be downloaded and installed on your computer. One of the best featueres of Piknik is you do not have to sign up and use it's photo editing features.
Picnik has been a huge hit with my 6th Grade Computer Class because of the dozens and dozens of effects you can easily apply to any image you upload. They can even add multiple effects and filters by layering them over the top of each other for even MORE cool customizations.
When my students are done, they can very easily save their edited photo back to their computer. Again, students do not need to sign up or use an email address to use this site.
There are Premium Features (which include many more filters and photo effects) that you can upgrade to by paying a small fee. However, we have found the free resources on Picnik to be more than enough for our photo editing needs!
If you ever need a quick resource for your students to crop, resize, flip, re-color, or add cool effects and filters to their photos, check out Picnik:
Below are some of the examples a student and I have created with Picnik: