This may help. I found this in "My Tips folder". If it doesn't work for you
then no doubt someone will soon be along with the correct answer! I copied
this many, many months ago and it is someone elses reponse to a similar
question.
If you are using PowerPoint 2002/XP and above,
You can go to the Slide Master /View / Master / Slide Master.
Insert the image through Insert / Picture / From File.
Send the image to the back, then click Slide Show / Custom Animation.
Click Add Effect / Emphasis / Transparency. Set the transparency of your
preference.
Best of luck.
MoMo (Maureen)