It sounds like your trapezium image doesn’t have a transparent background. The effect of this is the text will wrap to the border of the image and not to the content (trapezium) within the image.

Try this method to remove the background within Word.
First select the image, then change the Layout options to In front of text.
With the image still selected, click the Format, then the Remove Background option.
Use the handles to Mark areas to keep.
Click Keep changes or hit the Enter key when you’re done.

Now, with the image selected, click the Wrap Text tab, then the Edit wrap points option.
Now move the trapezium around to suit your poster and you can change the edit points also should you wish to.