In each of my post I set a feature image with a width of 800px.
This is fine, however if I browse to the blog index page, which shows all posts (the feature image + an excerpt), then I end up with a heavy page because there are 20 posts per page, so that's 20 images of 800px.
On the blog index page, how can I set the thumbnail to be of a smaller size. Actually it appears smaller but it is still using the same image as the feature image. So how can I use a smaller image for thumbnails ?