Image Comics' nature of allowing comic creators to retain the rights to the characters they publish with Image had led to many characters entering or exiting the Image Universe as the creators move from or to other publishers. Image co-founder and Savage Dragon creator Erik Larsen generally treats the Image Universe continuity as though each title exists in its own universe, and any crossovers between different creators' characters are events that occur simultaneously in each respective characters' universes.
When Jim Lee was an Image partner, his WildStorm properties such as WildC.A.T.s, Stormwatch, Gen¹³, Union, Deathblow, Wetworks, and Backlash regularly crossed over with other Image characters, but when he sold his studio to DC Comics, this ceased, and the WildStorm characters have since been incorporated into the post-"Flashpoint" DC Universe. Most of the comic characters in the Image Universe however are originally individually creator-owned from when the company was established.
There were several Image crossover mini-series in the 1990s, such as Shattered Image and Altered Image, but only Mars Attacks Image had any direct impact on the titles represented, as the Martians' attack was also shown and referenced many times in the Savage Dragon series.
In 2009, the Image Universe had its first major crossover in several years with Image United.
Superheroes in the Image Universe
Superhero teams in the Image Universe