An image is a false signal received on a frequency that does not actually exist due to poor receiver design.
For a detailed description of how and why this occurs see the article at Radio-Electronics.com linked at the bottom of this page.
Many people can receive control channels, repeater outputs, etc. in the upper end of the 700 MHz band around 790-800 MHz. These are images of 800 MHz stations. Whistler and Radio Shack scanners designed and formerly manufactured by GRE are particularly susceptible to 700 MHz images. See the forum thread GRE Scanner Spurious Calculator for more information.
Many older scanners used a dual conversion receiver with a 10.7 or 10.8 MHz IF. With these types of receivers images can be received at 21.4 or 21.6 MHz higher than the actual frequency.
The upper 700 MHz land mobile band is split into two 30 MHz wide sections. Mobiles use a +30 MHz offset.
- 746-776 MHz - Base (repeater outputs, mobile talk-around)
- 776-806 MHz - Mobile (repeater inputs)
See 700 MHz Public Safety Band Plan for the list of channels available for use by public safety trunked systems.
If you are receiving control channel data or repeater outputs in the 776-806 MHz range (in the US or Canada) you are receiving images.