remove links to external imgs (e.g. sensebox) and replace by local asset

Following the sensebox images moving servers, we realized that we shouldn't rely on these assets to be at certain URLs on other servers. Instead we should host the images ourselves (as long as we are legally allowed to use them).

@umut0 already agreed to kindly supply sensebox images for us.
Other devices will either need a generic image for now or we need to acquire them.