You can't use ftp on a wordpress.com blog so that's not relevant here.
You can reduce image size in the image editor but it does not reduce the original size of the image and it also succeeds in making the image quality poor. I'm afraid the only way is the long way that you don't like, and that is to resize your images before you put them in your blog and media library.
However, some image programs allow bulk editing. I use Photoshop Elements (which is a cheaper, cut down version of the full photoshop programs) and it has that feature. Only ever do that, though, on copies of your originals.
Two other consideration of original images being very large. One, certainly, is that you will lose space quickly (and you can buy a space upgrade but if you're always posting large images you'll eventually run out of that space too), but the other is that if you want anyone else other than you to see and enjoy your blog, resizing images will help them do that, as pages load very slowly the higher the images. Also with very large images, it's impossible to see the whole image without scrolling.