Integrate images into build

[Mischback]

While working on #31 one (unresolved) issue was, that the images are not actually part of the repository and thus, can not be considered/checked/processed during CI.

There are some possible solutions:

1. Integrate the images into the repository
   While this is possible to track binary files with git, it's not desired, as it may lead to a bloated repository, as every updated version of an image would result in an additional blob, effectively doubling the size, making the repo grow
2. Use git-lfs
git-lfs stores only references to the binary files in the repository and stores the actual files elsewhere. This is kind of desired, but may require an external git-lfs server, because GitHub has quite strict rules about storage capacity and bandwidth. This external git-lfs server may need massive storage capacity, as it will hold all versions of any binary asset.
3. Going old school and keeping track of the images manually in an ignored folder.
   This is the current approach. It only requires the images to be available locally. CI can not create a full build of the website!
4. Manual replication of git-lfs
   Store only references (and possibly hashes) of the files within a JSON file in the repository. During CI, download the files from a (known) source, compare the file hashes and create a full build.

Related

• Responsive Image Extension doesn't work for an image included in two different documents #120