By default when uploading extra files, via Muse, they are put into a created folder called 'assets' ... so you need to add that path to your link in able to use that asset ...
www.mydomainname.com/assets/picture1.jpg
Also, for best practice make sure you have ALL lowercase in your filenames ... no underscores '_' ... or hyphens '-' ... or extra dots '.' for that matter ... just one single worded filename:
picture1.jpg
Some browsers and hosting servers (depending on the OS) do not like CAPs ...
cheers,
Gem