When uploading media, images will be resized according to your Media Settings. You can regenerate all images you have already uploaded to your library by using the Regenerate Thumbnails plugin. Follow these steps:

1. Install and Activate Regenerate Thumbnails plugin.
2. Go to Tools → Regen. Thumbnails.
3. Click on Regenerate Thumbnails For All Attachments.
4. Wait for regeneration to complete. Done!