HEIC is a new image format created by Apple. It's smaller and more efficient than JPG. Unfortunately, support for HEIC is rolling out very slowly, so many of your devices likely do not support HEIC yet. This includes all Chrome-based browsers, regardless of operating system.
That means that HEIC images in your RemNote knowledge base will be unusable and appear missing on devices that do not support HEIC. This will include all desktop apps and the webapp on most devices. This is not an issue with RemNote – it is a limitation of Google Chrome (and the Electron tooling we use for the desktop app).