From 1984, Tàssies is an illustrator of children's books. His first international award was a Plaque BIB (Biennial of Illustrations Bratislava), in 1993,  for his illustrations of «El llibre dels bons amonestaments,» «The Book of Good Advice,» written by Anselm Turmeda in 1398, a work of Catalan medieval moral poetry. Tàssies gave the book a contemporary rebel slant, presenting a series of figures ready to be cut out.

Since the early 2000s, he has written text for several picture books. The first of them, «Volando del revés» («Flying upside down»), on the value of freedom and tolerance, was published in Spain, Mexico, and, in 2025, in Iran. «El nen perdut» («The Lost Child»), about the positive power of children's joy and hope even in dire situations like war and migration, won him the SM International Illustration Award in 2008 and, in 2009, his highest distinction, the Grand Prix BIB (Biennial of Illustrations Bratislava). The German Edition, translated by Jochen Weber, was released in 2017 (book and Kamishibai format).

Tàssies' work has been featured in several international solo exhibitions, including the 2010 edition of the Bologna Children's Book Fair when he presented his book «Noms Robats» («Stolen Names»), which speaks out against bullying at school. In addition to the Catalan and Spanish versions, «Stolen Names» is in Russia, Greece, South Korea, Taiwan, Japan, and Mexico; Edition Bracklo presented a German Edition in 2022 at the International Literature Festival Berlin. Several educational programs have been working with this book.

His opus «An» («If»), done with the Greek poetess Agathí Dimitrouka, is a claim for freedom. The book won the award for the Best Poetry Illustrated Book BookillFest 2018 in Serbia, the O Anagnostis Picture Book Prize 2019 of the critics magazine "O Anagnostis" in Greece, was included in The White Ravens list 2019 by the experts of International Youth Library of Munich, in Germany, and won The State Prize for Illustrated Children's Book of the Ministry of Culture of Greece in 2020. «If» was published in China in 2023 and Colombia in 2025.

Tàssies has served as a juror for the Bologna Children's Book Fair in Italy and the Biennial of Illustrations Bratislava in the Slovak Republic.

Tàssies has performed activities with children and young adults in Athens, Thessaloniki (Greece), Moscow, Krasnoyarsk (Russia), Jakarta, Bali (Indonesia), Bogotá, Puerto Estrella (Colombia, La Guajira)