Is there a way to crop images using a polyline or something similar

fresh user to librecad i am experienced with microstation, i would simply clip the reference raster to the polyline i have drawn (to crop the specific part of the image out that i want) but is there any way to do something similar in librecad

No, there isn't.
You have to crop the image or cut out the part you want with a graphics program outside of LibreCAD.
LibreCAD supports transparency in images, at least in png's, as I have found.