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Answer: If you have a large image file, the most effective way to reduce the image size is to save the file in a compressed format, such as a .JPG, .GIF, or .PNG file. If the original image file is uncompressed, such as a .TIF, .BMP, or .PSD file, the compressed version will most likely be 5% - 50% of the original file size.
Use the following guidelines when choosing a specific type of image compression.
- If your file contains many colors, like a digital photo, JPEG is your best option. While JPEG files use lossy compression, if you choose a compression ratio of 60 or less, the effects of the compression will most likely be unnoticeable.
- If your file is a simple image with only a few colors, such as a web button or black and white text document, the GIF format may be a good choice. GIF files can have a maximum of 256 colors, so colorful images may look grainy when converted to GIFs. However, if your image only has a few colors, the GIF format offers highly effective compression and is not lossy like JPEG.
- PNG compression works on all image file types and often offers the best image quality. However, it may not reduce the file size as much as JPEG or GIF compression. Therefore, PNG is a good choice when image quality is more important than the smallest possible file size. Screenshots, for example, are good candidates for PNG compression. Additionally, the PNG format includes an alpha channel, which means individual pixels can be partially transparent. This makes PNG the best choice for images that fade from 100% opacity.
Of course, you can also reduce the image size by shrinking the actual size of the image. For example, if your image is 6000x4000 pixels and you don't need the image to be larger than 3000x2000 pixels, you can reduce the image size by 50%. A 50% reduction in image size can reduce the file size by as much as 75% since the resulting image only has 1/4 as many pixels.
You can also reduce the image's DPI to save disk space. For example, reducing a 600 dpi image to 300 dpi may shrink the file size as much as 75%. However, if you print the image, the resolution will not be as high and the image will not be as sharp.
Most basic image editing programs can perform all of the above procedures. Some common image editors include Adobe Photoshop, Adobe Photoshop Elements, ACD Systems ACDSee, Corel PaintShop Pro (Windows only), and Pixelmator (Mac only).
