Hi Kate,
Thanks for posting your question to the forums and welcome. Can you share more details with us about what type of file this is and how you are trying to add the textures? Note that A360 is cloud based, so it will not know how to resolve custom images.
We do have an existing idea in our Idea board that you can add your vote and comments to.
A360 Idea boad - Customer materials for A360:
Hope this helps and again welcome to the forums. and Happy New Year.