It's very likely correct. The correct original quote should be:
Wenn es einen Gott gibt muß er mich um Verzeihung bitten.
It is a quote that supposedly appears on the walls of the jails of Mauthausen concentration camp, according to the documentary they show to people at the camp tours.
I have found many anecdotal confirmations of this.
I finally managed to track down a copy of said documentary, unfortunately in Italian, and it does make that claim. Given that is shown in the same concentration camp, I have no reason to doubt it's true.
From a transcript:
Nel carcere del campo, detto bunker, si impidocchiano i detenuti per esperimenti con gli antiparassitari, ma il carcere del campo serve anzitutto per isolare i prigionieri. La Gestapo del campo vuole estorcere confessioni e tiene i detenuti nelle celle, per bastonarli a sangue, per torturarli: per molti questa gabbia è l'ultima tappa della vita.
Mentre, di solito, sulle pareti delle celle ci sono scritte di speranza o testimonianze di uno spirito sveglio che si ribella alla prostrazione, qui troviamo l'ultimo monologo, segno di avvilimento, di disperazione, di morte imminente.
Dio mio perché mi hai abbandonato?
Piegarsi significa mentire
Se esiste un dio, deve chiedermi perdono
Queste scritte sono state ritrovate, dopo il 1945, sui muri delle celle.
In the prison of the camp, called the 'bunker', they give lice to prisoners for experiments with pesticides, but the camp's prison serves primarily to isolate the prisoners. The camp's Gestapo wants to extort confessions and holds the prisoners in cells, to beat them bloody, to torture them: for many this cage is the last stage of life.
While, usually, on the walls of the cells there are writings expressing hope, or testimonies of a wakeful spirit which rebels against bowing down, here we find the last monologue, a sign of discouragement, despair, of imminent death .
My God why have you forsaken me?
To bend means to lie
If there is a god, he must ask me forgiveness
These writings were found, after 1945, on the walls of the cells.
The documentary is available on YouTube: http://youtu.be/8r50t7148sA?t=19m20s
Given that the documentary is indeed showing the walls of the Mauthausen jails while this is described by they are (freshly) painted white, I think that the writings are not there anymore, so finding a picture will likely be almost impossible.
There is no proof that these were written by a Jewish prisoner:
- It is not claimed by the original source. The documentary doesn't say it was a signed message, nor that it was written in Hebrew.
- It was in the jails, not in the Jewish barracks, which excludes assuming authorship based on location.
- As mentioned in the comments, only a minority of the Mauthausen inmates were Jews.
Yet it is possible that some of the authors were Jewish:
Update: I've found on the official Mauthausen Memorial web site, a German and English translation of the documentary which is called "Rückkehr unerwünscht (return undesirable)". The interesting part is still at 19m20s.