Is it ok to "borrow" some images ?

My brother and I had an idea of a card game like Magic the Gathering ... He will mainly do the source code but he asked me to help him with ideas and graphic material ... The problem is that even dough we have experience in editing pictures we can only modify already made pictures or create some simple vector-like pictures ... Is it ok to take other pictures and use them as long as we mention that the game is freeware and that we are not the authors of the pictures ? (for example we chose some warrior character called Katsuie for our game brand ... later we found out that he was from another game ...)