Although Picasa 3.9 is not certified for operation on the Windows 8.1 operating system, you can install and use it without any problems. The default photo viewer in Windows 8 is the Modern UI Photos app, but you can configure your computer's operating system to use the Picasa Photo Viewer by changing the default programs settings. Finding the default programs settings is not difficult, but the procedure is different from Windows 7.
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Installing Picasa in Windows 8.1
Open the Picasa Web page and click the Download Picasa button. Although Windows 8.1 is not listed among the supported operating systems, you can install a fully functional copy of Picasa on it.
Click Browse and select the destination folder. Picasa 3.9 requires around 70MB of free disk space. Click Install to install the program on Windows 8.1. Usually, Picasa 3.9 is installed in less than a minute.
Enable or disable the various options on the Completing the Picasa 3 Setup dialog by checking or unchecking their boxes. You don't need to run Picasa to set it as the default photo viewer, so you can safely uncheck the Run Picasa 3 box. Click Finish to close the setup wizard.
If you leave the Send anonymous usage stats to Google option enabled, Picasa sends usage statistics to Google for analysis.
Changing the Default Photo Viewer
Move the mouse pointer from the upper right corner of the screen toward the lower right corner to display the Charms bar. Alternatively, press Windows-C. Click the Settings charm.
Click the Change PC settings button to open the PC Settings screen. You can quickly access other settings -- such as network, volume, notifications and keyboard settings -- by selecting any of the other charms.
Click Search and apps on the PC Settings screen to view all settings related to Windows apps.
Click Defaults to view the default apps, or programs, used by Windows to open various file types.
Click the default program in the Photo Viewer section to display a list of programs that can open image files. Select Picasa Photo Viewer from the list to use Picasa as the default photo viewer app.
Optionally, you can choose to open only specific file types with Picasa Photo Viewer by clicking the Choose default apps by file type link.
To change the default app for a file type, locate the file's extension, click the default app and select a different program from the list. For example, you can open JPEG files with Picasa Photo Viewer, but use the Photos app for GIF files.