I don't think there's anything terribly wrong with altering existing art to use as a poster for a local gig - if it get more people to the show, it's all good.
BUT, you didn't "just make a poster". You uploaded it to this site for all to see. 1) Over there on the left it says your name (I'm presuming) under the word DESIGNER, when you've not really designed anything here. 2) Don't be surprised when people pull you up on this after you knowingly uploaded the image to a public website which encourages comments and critique on peoples' work. 3) You could have at least fixed that thumb.