It depends on what you mean by "noticeable." In most cases, altering an image does not make it legal to use. You have to make the image unrecognizable.no you still need permission from who ever took the pic, but you can always change thing around with it so it wont be noticeable.
Try the Open Clip Art Library. It's all public domain clipart.ahh i have to pay for images too
is there a way to get free graphics that i can put onto shirts?