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.