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Although they come in many forms, the most common type of image macro is a photograph with large text superimposed in Impact font, using all upper case letters and coloured white with a thin black outline. 
One of the more famous image macros is "O RLY?" O RLY is often used on the internet as an abbreviation for the phrase "Oh, really?" Originally started with a snowy owl photograph (which is the classic O RLY image macro), it spread out over the Web quickly and was followed by other macros that convey a wide range of emotions. Another style of image macro that has amassed its own separate subculture is the "lolcat", a photo of a cat with a humorous and ridiculously misspelt caption.
The name "image macro" was first used on the Something Awful forums. The name derived from the fact that the 'macros' were a short bit of text a user could enter that the forum software would automatically parse and expand into the code for a pre-defined image, relating to the computer science topic of a macro, defined as "a rule or pattern that specifies how a certain input sequence (often a sequence of characters) should be mapped to an output sequence (also often a sequence of characters) according to a defined procedure."
Though modern-day image macros may not follow the literal concept of a macro, the name is still often used in accordance with their definition as described above.