1. A method comprising the computer-implemented steps of: while a plurality of users are participating in a conversation, receiving a trigger string; wherein text of the conversation is displayed within a shared conversation window displayed by at least a first messaging client and a second messaging client; wherein, within the shared conversation window, a sequence in which text is entered determines a position of the text, relative to other text, in the shared conversation window; wherein the first messaging client and the second messaging client belong to a messaging system; wherein the trigger string was submitted through a text input interface provided by the first messaging client; in response to receiving the trigger string through the text input interface provided by the first messaging client, causing at least (a) the first messaging client and (b) the second messaging client to generate an initial display of a string-triggered visual image; wherein the initial display of the string-triggered visual image exhibits at least one of: a behavior that is based, at least in part, on information obtained, after the trigger string is received through the text input interface provided by the first messaging client, from a service external to said messaging system, wherein the information obtained from the service includes textual data, and the textual data is obtained by the messaging system without involving the first messaging client and the second messaging client; a behavior that causes the entire string-triggered visual image to move from a first position to a second position, wherein moving from the first position to the second position changes the position of the entire string-triggered visual image relative to other previously-displayed elements of the conversation in the shared conversation window; a behavior that is based, at least in part, on the presence of one or more other elements, distinct from the string-triggered visual image, displayed by the second messaging client, wherein the one or more other elements were displayed by the second messaging client before receiving the trigger string through the text input interface provided by the first messaging client; a behavior that causes the string-triggered visual image to interact with one or more other elements, distinct from the string-triggered visual image, displayed by the first messaging client and second messaging client, in a manner that affects the display of the one or more other elements, wherein the one or more other elements include at least one of: textual elements of the conversation or another string-triggered visual image, included in the conversation, associated with a different trigger string than the trigger string received through the text input interface provided by the first messaging client; a behavior that is affected with a power level associated with the string-triggered visual image; a behavior that is based, at least in part, on results produced by a pseudo-random number generator; a behavior that is manipulated by user input received through an input device after the trigger string is received through the text input interface provided by the first messaging client and the string-triggered visual image is displayed by the first messaging client and the second messaging client; or a behavior that is based, at least in part, on non-image data extracted from a profile maintained by said messaging system for a user from whom said trigger string was received; wherein said computer-implemented steps of said method are performed by one or more computing devices.