Can I add an external or custom image to Chemix?
Short answer: Unfortunately, not at the moment.
Chemix aims to help you produce quality, consistent diagrams with correct real-life sizes and so each apparatus is hand-drawn and added manually. This ensures the look is consistent and that items are scaled properly relative to each other.
Using an external editor
If the above is not an option for you, an alternative is to draw most of the diagram in Chemix, save it and then load it in another drawing application where you can finish the diagram by drawing items not available in Chemix.
Good drawing applications are: Microsoft Paint, Microsoft Word, Google Docs or https://www.photopea.com/