Accessing multiple images sizes from hosting

When I save an image, I get multiple sizes. For example if the original image was 900×600 there might also be a 450×300 version and a 300×150 version saved on my hosting (GoDaddy).

Often, I use different sized versions of one image for things such as the primary image on a post and the featured image on masonry grid (trying to save on load time). I’m having trouble though with always accessing the different files…???

Since the most recent WP update, I can only seem to add the largest file as the featured image. (Its especially slowing down my blog page). While I can edit the primary image size in the text of a post, I haven’t found a way to do so for the featured image. I know the files are there in my FTP. Does anyone know how I can add these image sizes without uploading them again? or am I stuck?