Working with images:
Reducing graphic data

This guide is just a simplified aid for moments when you need to reduce the size of an image file quickly and without lots of philosophizing.

FILE  REDUCTION  PRINCIPLES

Images can be reduced manually using a photo-editing program.  Any of the following methods or a combination of any of them can be used.

Point resolution: 
For example, reducing the number of pixels from 100 x 100 pixels to 50 x 50 pixels will shrink the image by 75%.

Color resolution: 
Black-and-white / grayscale images have one color channel (shades of grey).  Display images usually have three color channels (e·g·, RGB system stores shades of red, green and blue).  Print images have four or more channels (e·g·, the CMYK system mixes information about cyan, magenta, yellow, and black).  In addition, the channels can have different depths (it makes a huge difference whether you use 256 shades or millions of colors).  By reducing the number and depth of channels, it is possible to achieve a surprising drop in data volume.

Compression and quality: 
For example, if an image has a million white points, some image formats (such as classic bitmaps) mechanically store the same white-encoding information in a million memory locations side by side.  Wiser formats will store such information only once in a single memory location, and add a brief note that this stored information needs to be repeated a million times.  The space saving is enormous!  There are other methods of "shrinking" or "compressing" images.  Some of these preserve the full image quality, others are lossy (they change the image a little to a simpler one and some details may be lost).  In lossy compression, the user specifies the desired resulting quality (for example, on a scale of low-medium-high, or as a percentage, etc·; it depends on the program used).

Hidden file descriptors: 
You typically can't see the so-called metadata when you view the photos.  However, many photos internally remember the camera brand, aperture used, illumination, and many other pieces of information.  These take up space.  Text and numeric data is usually short and saves little space when you delete it, but sometimes even one bit will help you to meet the size limit.

Resolving all of the above aspects of an image can be cumbersome and may require the use of multiple programs.

One possible solution is to use online converters.  They don't always tell you which method has been used.  But the important thing is the result:  In one or a few clicks, you have a new image of the desired size in a few seconds. 

At the page top are links to several free servers.