Hi Steven,
you cannot prevent this - the texture effect will show up on any image drawn on the canvas.
The effect is more noticeable with some textures than other - but it is part of the way the software works.
If you would like to see a feature to 'Remove canvas texture from images' you can post a new question and select 'Ideas and feature requests' in the 'Post topic in' section.
-Matt
Steven Sauve