In general each and every question should be self contained in text (*1). Every other media is usually neither searchable nor always displayable. Also, describing picture/video/recording content in a textual manner does allow to put emphasis on components relevant on the question.
If other media gets embedded, they will always be foreign to SE and rely on a the continued existence of its (third party) service. Using the offered service to embed pictures must be included here, as Imgur falls into the same category. In general I can see no difference between Imgur, Youtube or my mother's NAS (*2). In fact, even services with a clear long term mission, like Archive.org, can vanish on days notice.
As a result, the only guideline that can be given is the same for any question: Describe in you own words what the question is about - and do so as well for all related material, linked or not.
More important than requesting media to be stored on certain services would be to make sure it's accessible for the general public.
Naturally this doesn't work for questions which are about unidentified content of such media - like the example is a perfect specimen for. After all, a description could only read 'a typewriter, another one and two more .. and the same for computers that followed', as those were exactly the facts the poster didn't know at all. And let's be honest, describing a computer or a typewriter (or any other complex device) by someone who doesn't know what's special, is funny at best - usually but rather confusing.
Sure, selecting single frames from a video is a great way to handle it, but as well time consuming. I would not request such by default from anyone asking. Not even from later edit (*3).
*1 - And here, to be safe, as far as possible in plain (7Bit/ASCII) encoding.
*2 - Having said so, there is of course a certain expectation that some services may last longer than others ... still, my Mom's NAS has good chances to outlast Youtube, if only for the fact, that no one touches it at all :))
*3 - This does not mean I don't appreciate the effort of later editors to enhance a question's readability.