How do I reduce the size of photos for online use?

Reduce Photos for Faster Downloads

The Photoshop Image Size dialog box is shown over an image of a lighthouse.
The Photoshop Image Size dialog box is a great place to start trimming the fat off of your images.

Question: How do I reduce the size of photos for online use?

Answer:
Before going to your intended size, you first want to crop your image to remove any unnecessary portions of the picture. After cropping, you can change the overall pixel dimensions to go even smaller.

All photo editing software will have a command for changing the pixel dimensions of an image. Look for a command called "Image Size," "Resize," or "Resample." When you use this command you will be presented with a dialog box for entering the exact pixels you wish to use.

Other options you may find in the dialog are:

  • Resample - If the image is shot with a DSLR the odds are pretty good that it has a resolution of 300 pixels per inch(ppi) . If this is the case reduce the value to 72 or 100 and the physical dimension of the image will also reduce. You never increase the value. You decrease it. 
  • Constrain proportions or keep aspect ratio - You want this option enabled.In Photoshop ,click the chain to link the width and height values. It prevents the image from being stretched and distorted. When this option is enabled, you only need to enter one value--height or width--and the other value will adjust automatically.
  • Change the resolution value before changing the Width and Height values. Changing the resolution to a lower number - called downsampling - will also reduce the width and height values.

File Format is key.

Online images are either in the jpg or png formats.

The png format is a bit more accurate than the jpg format but they tend to have a slightly higher file size. If the image contains transparency then you need to use the PNG  format and to be sure you select the Transparency option.

JPG images re regarded as "lossy". They are so small because because areas of contiguous colour are, and this is a very loose description, grouped into a single area reducing the need to remember the colour of each pixel in the image.

The amount of compression is determined through the use of a Quality Slider in Photoshop. The values range between o and 12 meaning the lower the number, the lower the file= size and the more information that is lost. A value of 8 or 9 is common for images destined for the web.

If you are a Sketch 3 user,  you get to set the Quality when you click the Export button in the Properties panel. You will be presented with a Quality slider that ranges from 0 to 100%. A common Quality value is 80%.

When choosing the compression level, keep quality in the medium to high range to avoid compression artifacts.

Never recompress a jpg image. If you have received an already-compressed jpg image, set its quality to 12 in Photoshop or 100% in Sketch 3.

If the image is small or contains solid colors consider the use of a GIF image. This is especially useful for single color logos or graphics containing no shades of colour. the advantage here is the ability to reduce the number of colours in the colour palette which has a major effect upon file size.

Never, ever resize and overwrite your original file!
After sizing the image, be sure to do a Save As so you don't overwrite your original, high resolution file.

here are a couple of tips:

  • You want to shoot for a file size of 50 to 250 KB per image. Go small if you will be putting several files on the same page or sending them in one email.
  • Try not to exceed 1000 KB per Web page for the total of all images.

This may sound like a time consuming process, especially if you have a lot of photos to share, but fortunately most of today's software has made it easy to size and compress a batch of photos very quickly. Most image management and some photo editing software has an "email photos" command that will resize and compress the images for you. Some software can even resize, compress, and generate complete photo galleries for posting on the Web. And there are specialized tools for both of these tasks--many of them free software.

Update by Tom Green

Batch Resizing