The phone activity that uses the most data is video streaming and audio streaming like watching YouTube videos, TikTok videos etc and streaming music or audio.
Video streaming on your phone when streaming HD videos can use as much as 3 GB of data per hour and streaming or watching 4K videos can use as much as 7 GB of data per hour.
Video conferencing like through FaceTime and Skype can use hundreds of MB of data per hour.
Navigation apps like Google Maps use data constantly updating and downloading details and live traffic information.
Viewing photos on your phone or computers hard drive or USB thumb drive does not use data.
However when you view photos from your phone or computer through the internet it it does use data as the photos have to be downloaded from the internet to your phone or device to allow you to see the photos.
For example viewing photos on Instagram, blogs, google images etc will use data on your phone or computers internet service.
Just like watching YouTube videos on your phone or computer will use data to download and play those videos on your device.
Although you're not directly downloading the videos or photos to your computer or device, the data is still being downloaded to your computer or device as you watch the videos or view the photos.
The amount of data the viewing photos on the internet or watching videos on the internet uses, will depend on the size of the photos and size of videos you watch or view.
Viewing photos uses a lot less data when viewing the photos on the internet and the average photo uses around a few MB of data although some larger photos can use more and videos can use around 500 MB or less and sometimes more.
But like mentioned before, viewing photos or videos that are saved on your hard drive or device without it coming through the internet does not use any data.