Sometimes the DMG won't run properly from the Downloads folder, but the Mac will happily install from a USB drive. If you have a working DMG (or a "fixed" one that mounts but won't install), you can restore it to a USB flash drive.
The most reliable, community-verified source for legacy Apple software is the Internet Archive (archive.org) . Navigate to the Mac OS X Lion page on Archive.org.
hdiutil attach -noverify -ignorebadchecksums
Shortly after the 10.7.2 update was released, users noticed a significant change. The InstallESD.dmg file, which was easily accessible inside the Lion installer bundle before, seemed to vanish. Apple, perhaps intentionally, changed the installer package's structure or hid the file deeper to discourage users from bypassing the App Store. Many forum posts from late 2011 ask, "Where did my Lion DMG go?".
: General fixes for Wi-Fi reliability, Safari stability, and better compatibility with Microsoft Exchange servers. Apple Support Minimum System Requirements
