Controlling reflections

Hi there,
Im fairly new to blender, and I need help with a project. Im trying to render this still image, of a prism-like crystal in front of a flat waterfall image. I have made a scene inside a room, and applied my image to several planes in order to place the crystal in the context of the image behind it. However I am getting a few strange reflections that I cant figure out how to fix. Like the ones in the triangle areas

as you can maybe tell from the pictures I placed the image above below, in front and behind, and also applied the image as a texture to the cube “room” surrounding the composition. Also Im wondering why the image is stretched into these lines on the cube when applied as a texture

One more important thing I need help with: what is the best way the export a rendered still image, is “save rendered image (F3)” the best quality?

Thanks for any help, I appreciate it! :slight_smile:

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