PolyFrame lets you display an image or a flat color inside a polygon shape (triangle, pentagon, hexagon, heptagon, or octagon), with an option to round the corners. Just drop in the element, pick a shape, and you are done.
No custom CSS or workarounds needed. Works with both dynamic and static images. It is completely free.
See Live Demo or view it on Bubble Marketplace.
Let us know if you run into any issues or have suggestions.