Image retention and burn-in are two different things. This article will explain the difference.
"Image retention" is a phenomenon that only occurs with plasma display panels. If there is a stationary object or a graphic image that remains on the screen for several minutes, image retention will be a distinct shadow of the image that persists when the picture changes. The shadow will typically fade away after several minutes of normal viewing. The degree to which image retention is noticeable varies with make and model, as well as brightness, cell drive, and picture (or contrast) settings. Most late model plasma TVs perform very well in this regard, manifesting almost negligible image retention. Many TVs have optional features for minimizing this effect (consult your owner's manual).
Another related issue is where the retained image manifests itself as randomly flickering pixels that linger after the picture changes. There are two reasons why this can occur. One is if the voltage levels or waveforms on the power board that drive the panel are not properly set. Adjustments to these voltages should only be done be a qualified professional. The other reason is if the panel has deteriorated due to aging. Sometimes plasma panels will age prematurely if the voltages are incorrectly set for a long period of time. Panels with aging issues cannot be fixed; they can only be replaced.
Finally, if any flat screen TV is left on for a very long time with a stationary object or graphic image in the picture, a permanent shadow of the image can be retained, or burned-in on the screen. When the shadow never goes away, this is called "burn-in". Plasma panels are more prone to burn-in. It takes much less time for an image to burn into a plasma panel (sometimes less than 24 hours, depending on make and model) than it would for an LCD TV (which might take several days or weeks). Panels with burn-in issues cannot be fixed - they can only be replaced.