In typical digital photography applications, the larger the file size the clearer and sharper the photograph. However a large file size isn't always preferable. If you are sending an image via email, it is likely that your email provider has set a file size limit. It is therefore preferable to have a smaller version of the same photograph to send out via email.
Double-click on the thumbnail version of the photograph you want to resize. The photograph will open in your default photo viewer or photo-editing software.
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Confirm the current file size. Right-click on the image and select "Properties." This tells you the dots per image, or "DPI" setting and file size. The file size will be denoted in bytes, either in kilobytes or megabytes. 1 kilobyte, or "1KB" is 1,024 bytes and 1 megabyte, or "1MB" is 1,048,576 bytes. Make a note of the file size. High quality photographs are typically saved as large file sizes measured in megabytes.
Double-click the photo again and select "Open with" and select your preferred photo-editing software, for example "Photoshop" or "GIMP." The photograph will automatically open in the program.
Calculate what percentage of the current file size 350KB represents. For example, if the current photograph is 3.5MB, that's equivalent to 3,584KB. So 350KB is approximately ten percent. As a guide, 1KB is 2 to the 10th power and a megabyte is 2 to the 20th power.
Click the "File" menu and select resize. You will typically see an option to manually resize the image by stretching it with your cursor or to resize it by a percentage. Resizing with the cursor is suitable for quick visual adjustments, for example if you are sizing the image to fit a frame on a page. But it is not an accurate method of resizing to a specific file size. Enter the percentage by which you want to resize the image. In this example, to make a 3.5MB file 350KB, type in "10" underneath the percentage symbol. If you need to increase a very small photo from 35KB type in "100" to increase it by 100 percent.
Click "Save As" and rename the file to distinguish it from the original. It's important that you don't mix up the original with the smaller file, because they are suitable for different applications. The original is likely to be too large to send via email. Email providers typically limit the size of attachments, for example the limit for Google Mail is 25MB and for Yahoo and Windows Live it is 20MB. The new, smaller file is too small for print. The small photo will also have a lower resolution, or "dots per inch," which means any application that automatically resizes it will make it blurry and distorted. If printing an image, consider dots per inch, or "DPI", rather than file size. The typical minimum DPI when printing an image is 300 DPI.