Oh wow that’s not good (but also a bit funny).
So the templates ship with some (very) barebone CSS which lives in _css/franklin.css, in there if you look at these lines: Research/franklin.css at 0342704396812c4345230d365595b8e5a7382d43 · CrucialFlow/Research · GitHub there are fairly catch-all rules for images. So if you just use the basic  it will fall in the first generic rule which makes the image 70% of the visible width (which looks horrendous in your case).
So to avoid this you can just remove those lines, maybe add your own that look like the .img-small one so that you could have specific styles for given images.
Finally you might want to have a look at this issue where there is more discussion on how you can generally specify image dimensions building your own commands which can be useful.
PS: one other small thing, the templates ship with an initial favicon.png that is located in _assets/ , it’s an image of an iguana. I initially thought that was quite funny but now I realise that people may forget its there and end up with that iguana for a fair while. I really need to remove it from default templates. Anyway you can either:
- change that
favicon.pngfor another one that would make sense for you - remove the
faviconline in_layout/head.html - (leave the iguana)