Who can give me some "pro tips" on creating a PowerPoint presentation?

I’ve made several, and taught myself a thing or two as I went along, but I would appreciate some expert advice, for example, how long, on average, to leave one picture on display. What fonts to use. What transitions to use. Colors. That kind of thing.
And do you make your own formats, or do you use the preset formats (I’ve always created my own.)

The theme is to showcase Habitat houses we’ve built in this community. Today I got access to a little over a million pictures from 1993 to present! I’m exaggerating but it’s at least a thousand so I need to start culling editing.