EasyGals automatically generates a thumbnail gallery for posts with images attached to them.
Usage of this plugin is simple, you may configure it by going to 'Options –> EasyGals'. To use EasyGals you simply write a post and upload some pictures with it. Before publishing it, you'll need to add a marker in your post at the position you would like the gallery to be displayed. When you're using the visual rich editor you'll need to go into the tab that says 'code', and type <!-–easygals–-> where you would like the gallery to appear.
A gallery that's generated by EasyGals is contained within a div with a class of 'easygals'. The styling can be done in the EasyGals options screen.