Yes - this is how we do it. I made new PowerPoint templates for our pastor and he creates them in PowerPoint (easy with templates) and then they are exported to PNGs. He definitely couldn't use (or afford) Photoshop.
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Putting slides into a PDF file does not seem of much use (unless I am misunderstanding how the other poster does it) because the service slides are interspersed with songs.
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The one thing that annoys me about OpenLP when you have a batch of service slides as images is that the 'pointer' does not progress through each slide in the image group, only group by group. This makes it harder to know where you are up to in a sequence of service slides.