Make image and other asset names consistent

[mishaschwartz]

Right now, images and other assets have uninformative and potentially confusing file names. This is because they were downloaded from another source and then added here without making any change to the original file name.

This means that we have files named things like: usgs-eVcfzGhVpYc-unsplash.jpg and MEIE_-Avril_2014-300x130.jpg that provide no indication what they contain or are used for in the context of this project.

We need to establish a naming convention for these files and change the names of all image and other assets files in this project to follow this convention.