Can I reduce App download size by reducing bit depth of images?

Our application (iSew) has a number of applications within it. The download times are too long and I have discovered that a number of background images have 24 bit depth. I have reduced this to 8 bit to make them smaller and they still look fine. The size change (only for about 8 background images) was from 17.5Mb to 5.6Mb!

However, will this change the size of the download?

I am concerned that possibly the images are scaled to a standard bit depth which might mean that there will be no change to the download size.