Version 10.2.5 inherits the features introduced in the 10.2 update cycle, which focus heavily on content:
Solution: Check the file size. Legitimate iMovie 10.2.5 DMG should be approximately 3.2–3.4 GB. If smaller, redownload. Also verify checksum if provided by source. imovie 10.2.5 dmg
Apple’s Official Support Downloads (Rarely hosts old versions) Version 10
macOS Installation USB creator tools (Indirect) macOS Installation USB creator tools (Indirect)
Backup from another Mac (Best method)
While you can still find iMovie 10.2.5 .dmg files on third-party sites, be aware:
Solution: This is rare for DMG drag-install, but if you encounter it, restart your Mac and try mounting the DMG again. Corrupt disk cache is usually the culprit.