The paintings of Roy Lichtenstein, who was born 94 years ago today, are some of the most iconic examples of the pop art movement of the 20th century. Through appropriating the techniques and styles of comic books, a product considered decidedly juvenile and frivolous, Lichtenstein found a way to unite "low" and "high" art in captivating compositions.

However, as Lichtenstein enjoyed worldwide acclaim and his works continue to sell for tens of millions of dollars two decades after his death, comic book artists have voiced frustration at having their creations and genre appropriated with little credit and no money.

Some of these criticisms come over what is perceived as plagiarism by Lichtenstein. "The fact remains that Lichtenstein was simply reproducing the work of the original comic artists, without adding much," comics artist Marc Ellerby told the Guardian on the occasion of a large retrospective of Lichtenstein's career at London's Tate Modern in 2013. While expressing respect for Lichtenstein's talents in works like "We Rose Up Slowly," Ellerby also felt that his paintings of single frames hardly did justice to the narratives of comic books that could be hundreds of pages long.

"Lichtenstein did no more or less for comics than Andy Warhol did for soup," echoed Art Spiegelman, creator of the graphic novel "Maus," said in a 2007 interview.

Comics artist Dave Gibbons, who famously drew the graphic novel "Watchmen," has made even more pointed criticisms of Lichtenstein's work. "I’m not convinced that it is art," Gibbons told the BBC in a 2013 interview. "A lot of Lichtenstein’s stuff is so close to the original that it actually owes a huge debt to the work of the original artist," he added, comparing it to music where artists usually credit the original creator of a song even when parodying it.

To shine more light on the source material behind Lichtenstein's work, comics enthusiast David Barsalou has spent more than three decades painstakingly tracking down the original strips that the artist painted after in a project called "Deconstructing Roy Lichtenstein."

A more personal comment on Lichtenstein's relationship with the comics industry comes from veteran artist Russ Heath, the creator of the original comic panel that Lichtenstein used in his famous painting "Whaam!" In 2014, Heath, who is now 91 years old, drew a short strip about his experience having his work appropriated by the artist while struggling to make ends meet.

Now semi-retired and living on a "fixed income," Heath says he survives with the help of the "Hero Initiative," a charity that supports comic creators in need. "Roy got $4 million dollars for it," Heath said of "Whaam!" "I got zero."