Hi,
Thank you for your enquiry.
It is not currently possible to apply triangular borders to images or other objects. However, if the page background is a solid colour, you could mask out the top-left and top-right corners of the image using vector shapes filled with the same colour as the page.
For example, in the following screenshot, I have aligned two vector squares to the top-left and top-right corners of the image and have then rotated the squares to form a triangle:
If I now set the page background to match the colour of the triangles, the following effect is achieved:
However, the best solution would be to open the original image into a dedicated image-editor (such as Jasc Paint Shop Pro or Adobe Photoshop) and use the shape-selection tool (sometimes called a cookie-cutter tool) to produce a triangular section of the image. You should now be able to save this as a PNG image with transparent background and import this into Opus using an image object.
Kind regards,