seashore
Seashore is now on the App Store !!!
Please add your positive review/rating there (I believe people are trying to tank the project by posting clearly incorrect reviews). Also, if you use it for your website, business, whatever, giving credit to Seashore and a link to the App Store will help the project survive and prosper.
If you encounter a problem, please file an issue here so I can get it fixed! Leaving bad reviews without trying to help is not cool.
There are simple tutorial videos available here.
The current version requires 10.9+. If you are on an earlier OSX version, I suggest downloading the 0.6.6 binaries from seashore.sourceforge.net. Most likely you need to download the "hfs" version in order to open on an older Mac.
Seashore was originally located at https://sourceforge.net/p/seashore and was the brainchild of Mark Pazolli.
This is the currently maintained version.
If you are looking for an free, easy to use, image editing application with "layers", this is the one.
Here's a nice article (with screenshots) - a bit outdated - on the Seashore project at Libre Graphics World