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 gallery plugin might be used in another joomla component.
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/
You can also choose the same width and height and cropping on/off for the main gallery thumbnail, and for the lightbox thumbnail. So for example, in the profile settings, if you set the main image thumbnail to 300 wide and 200 high and yes for crop to the same size, then you can also set the lightbox thumbnail to 300 wide and 200 high and yes for crop to the same size. Then just 1 thumbnail image will be made, and it will be used in the main gallery and the lightbox gallery.
You can also choose the same width and height and cropping on/off for the main gallery image, and for the lightbox image. So for example, in the profile settings, if you set the main image to 1200 wide and 800 high and yes for crop to the same size, then you can also set the lightbox image to 1200 wide and 800 high and yes for crop to the same size. Then just 1 image will be made, and it will be used in the main gallery and the lightbox gallery.