If you choose the original option, every photo and video you upload will count towards your quota. As stated in the Google Photos help article, using the "original" setting counts against your free storage, regardless of your photo size.
Whether there's a cost depends on your settings. You can choose between 2 storage sizes for backing up your photos and videos:
High quality: Unlimited free storage for photos and videos that might be reduced in size or resolution.
Original: Counts against your Google account's 15 GB of free storage for photos and videos that are stored exactly as you captured them.
Google help also points out that
If the camera takes photos with 16 megapixels (MP) resolution or lower, most stored photos will essentially look the same using either storage option.
You can also change your mind at any time, however -
Changing the setting will affect storage sizes going forward and won't resize items you've already stored.
Sources: https://support.google.com/photos/answer/6220791 and https://support.google.com/photos/answer/6193313