Idea of Upload to Cloud Icon
The icon for upload to cloud is a cloud with an arrow pointing up. Since you are loading the images on a cloud service the arrow pointing up represents the images going into the cloud system that keeps the pictures out of your device but still easily accessible.
This page shows the different variations of the Upload to Cloud from an Icons8 icon pack. Feel free to resize and recolor the icon, and then download the icon in PNG format for free.