Use Picasa 3 on your Windows 7 or 8.1 computer to resize your images and change their quality. The file size of a lower-quality image is smaller than that of an image that contains a lot of fine detail. When exporting images from Picasa to a folder on your hard drive, you can set a maximum size in pixels. The software changes the height or width -- whichever is longer -- of your photo to meet your requirement. Picasa resizes the shorter dimension automatically, preserving the aspect ratio of the image. Picasa also has an online version: Picasa Web Albums.
Although Picasa 3.9 is not certified as a Windows 8.1 app at the time of writing, it can be installed in 8.1 and works in Desktop mode. Open the Picasa Web page, click the "Download Picasa" button and save the installer on your hard drive.
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Click "Browse," select the destination folder and then click the "Install" button to start installing Picasa on your computer. Click "Finish" to complete the setup.
Launch Picasa and select to search the My Documents, My Pictures and the Desktop folders, or the entire computer, for pictures. Click "Continue."
Select the folder that contains the images you want to resize from the left pane and then select the images from the right pane. To select all images in the folder, press "Ctrl-A." To select several images, hold "Ctrl" and click each item.
Click "File" and select "Export Picture to Folder" to open the Export to Folder dialog box. Alternatively, press "Ctrl-Shift-S" to open the dialog.
Click the "Browse" button and select the destination folder. Type a name for the sub-folder into the "Name of exported folder" field.
Click the "Resize to" radio button and type a new size -- in pixels -- into the field. Alternatively, drag the slider to select one of the predefined sizes.
Select the image quality from the "Image quality" drop-down box. The Maximum option preserves fine detail but generates large files. The Minimum option generates the smallest file size but some of the quality is lost.
The Automatic option preserves the original quality, while the Normal option provides a good balance or quality and size. To use a custom quality setting, select "Custom" and use the slider to select the quality.
To add a watermark to selected images, check the "Watermark" box and type the watermark into the field.
Click "Export" to resize the images and save them into the folder you selected earlier.
You can import your images from Picasa Web Albums by clicking "File" and selecting "Import from Picasa Web Albums." Select the albums you want to import and click "OK."
If you send images from Picasa to other people via email, resize the pictures automatically to avoid attaching large files to your emails. Click "Tools," select "Options" and then select the "E-Mail" tab. Select a size using the "Size for multiple pictures" slider. To resize single images, click the "Same as multiple" radio button. Click "OK."