Can you save as (or export) to PNG with transparent background in LibreOffice Impress ?

edit

Yes, you can...

Use the Pipette tool (Tools menu; so called "Color Replacer") to change some color(s) into transparency, then (menu File) export selection as png file and accept transparency property. "Interlaced" is only useful for websites.

Cheers

PS: For more information ask more details. ;-)