I found 2 problems with image insertion/linking:
1. I never succeeded linking/inserting an .eps format image. That also means that I can't insert a vector image into TeXmacs, and I won't be able to get high quality vector image output when exported to a PDF file.
2. When linking/inserting a .png file, the image cannot be saved with transparency, or else it will be wrongly displayed when exported to PDF.