 without the ./ works for me: https://gitlab.com/cirosantilli/test/blob/bffbcc928282ede14dcb42768f10a7ef21a665f1/markdown.md#image
I have opened a request for this to be allowed at: http://feedback.gitlab.com/forums/176466-general/suggestions/6746307-support-markdown-image-path-in-current-directory-s , but it entered into Internet void when GitLab dumped UserVoice.