Why is Jesus portrayed and remembered in a state of suffering? My observation of Christianity is that Jesus Christ is portrayed, worshiped and promoted in his state on the cross: suffering, crucified, having wounds. I don't see what sense this makes. This seems like a form of sadism.

Why is he not portrayed healthy? Why aren't his good deeds before the last supper remembered instead?
Catholics portray, worship, and promote Jesus in all of His states of being, actions, words, etc.

A strong emphasis is placed on his suffering and death on a cross because it was in this act that He atoned for the sins of all humanity.  This is the single most important thing He did because without it, we have no hope.

Paragraph 623 of the Catholic Catechism states:


  By his loving obedience to the Father, "unto death, even death on a cross" (Phil 2:8), Jesus fulfills the atoning mission (cf. Is 53:10) of the suffering Servant, who will "make many righteous; and he shall bear their iniquities" (Is 53:11; cf. Rom 5:19)


It would be highly mistaken to think that Catholics only focus on the suffering of Christ. The Catholic Church teaches that in rising from the dead He confirms who he is and the work that He accomplished by His death.

Paragraph 651 of the Catechism states:


  "If Christ has not been raised, then our preaching is in vain and your faith is in vain."521 The Resurrection above all constitutes the confirmation of all Christ's works and teachings. All truths, even those most inaccessible to human reason, find their justification if Christ by his Resurrection has given the definitive proof of his divine authority, which he had promised.


I'm sure with a little investigation, many examples can be found of Catholic images, prayers, etc. the are devoted to the healthy, happy, resurrected, glorious, etc. natures of Jesus Christ.