This post list some useful resources and sites that offer presentation templates, in different formats like OpenDocument (odp) or PowerPoint (.pptx).
Presentation Templates
Slidesgo
https://slidesgo.com/
Company in Spain. One of the co-founders sold Panoramio to Google.
SlidesCarnival
https://www.slidescarnival.com/
Free images
FlatIcon
Freepik
https://www.freepik.com/free-photos-vectors/presentation
It contians both presentations and images.