Depending on the type of editing, there is different applications I use.
For simple batch compressing / resizing I use XnConvert. It can also add watermarks and has some other nice features too.
For general image editing and manipulation I use (and always have used) Photoshop, although since the release of creative cloud and it's desktop app I noticed my iMac is very slow to startup as are most apps. This was not the case with the creative suite and I have read similar complaints on the adobe forums.
Gimp is supposed to be quite good for an open source tool but I have not yet had the chance to play around with it.