Distorted transparency issue!

Hiya ppl…

I created an image inphotoshop and saved it will transparent areas surrounding. I then open the psd file in corel photo-paint and alter the size then save as .cpt file. i then import the image into CorelDraw X3 and all is well.

Now, when i eport it to a pdf i find that any vectors/graphics/etc. that overlap the transparent area of my image become distrorted slightly (look pixelated, possibly enlarged slightly). this can also be seen when i print it out…

help please!
why could this be happening?