High Efficiency Image Format is a file format adopted by Apple in its latest operating system - macOS High Sierra. At this moment, HEIF is not supported by Picaboo.
macOS High Sierra users will need to first export their photos as JPEGs (or PNGs) from the Mac Photos app.
To do so, select the photo you wish to export and click on File > Export > Export 1 Photo. It is important to select the first Export option and not the "Export Unmodified Original..." option. The latter option will export the photos as HEIC files, which are not supported by Picaboo.
You can select more than 1 photo to export at a time by holding the Command button on your keyboard.
In the Photo Kind dialog box, select the desired image format - either JPEG or PNG.
Color Profile should be set to sRGB as any other profile selected may impact the color reproduction.
Once the photos are exported as JPEG or PNG image files, you will be able to import them into your Picaboo project.