Tonnis' works are "supported with psychological knowledge"[1] His earliest drawings reflect his interest in psychoanalysis and psychopathology such as, catatonic rigidity or the postnatal psychosis depicted in his 1980–85 collection.

His large scale triptych "Frost" is "a material image in harsh black and white which depicts a literary landscape of snow and ice in different viewpoints [...] a picturesque transformation of Thomas Bernhards 1963 novel".

This "experimental collage of different media and arts" has been a dialogue with Heinrich von Kleist's play Penthesilea, directed by Hans-Jürgen Syberberg in 1988 with actress Edith Clever.

[8] With the mural of a golden cross on black and violet ground—divided into pixels—Tonnis has been one of 36 international artists who designed the "Pixelkitchen" in January 2013, a tiled room of 177.2 inches height at the Günes Theatre in Frankfurt.

In his glowing hand he held a bouquet of blue flowers and used them to write two passages from Novalis' novel Heinrich von Ofterdingen on a large scale horizon.

Alongside these works stand the video series of "Dreams", "Electrical Pictures", and animals—exhibiting a pop, surreal pictorial language, often humorously staged.