All the images that are visible in the frontend galleries are stored in images/igallery/resized/.
If you change some settings in the profile, the extension will create new images at the new size, or with the new watermark etc.
The old images will not be deleted. The reason why the old images are not deleted is that the module or plugin might need them, and the gallery can not work out in advance where the plugin/module are being used, and in what way. For example the module might be a random image module, or the plugin might be used in K2 or Zoo.
If you have changed the profile settings a lot, there will be unnesscessary images stored in images/igallery/resized/. To save space, you can delete the images/igallery/resized folder. The gallery will remake this folder, and remake all the correct folders/images inside this folder, when the frontend galleries are viewed. The first view of each frontend gallery will be slow as the images are being remade.
Please don't delete any of the other folders, such as images/igallery/original/