When uploading galleries the images have to be in a zip file and you cant upload another picture into the gallery later on without rezipping the images with the new one and upload them all together again. Neither can you remove just one of the images.
This plugin, as well as the Pro version, is very poor.
The Free version of the plugin really does what it says in its name. You just upload a bunch of images and it displays them in a nice grid. Not to be confused with the Pro version which provides a lot more features.