While an iMovie 10.2.5 DMG technically exists (people extracted it from the App Store cache years ago), you should not download a random DMG from the web.
Your safest bet is always the Mac App Store > Purchased > Last Compatible Version. imovie 10.2.5 dmg download
If you previously had iMovie 10.2.5 installed and lost the application: While an iMovie 10
If you ever downloaded iMovie 10.2.5 before on an Apple ID: If you previously had iMovie 10