Graphic makers permission?, Graphic makers permission? Options

[Maccabee]

Hey there are some really cool backgrounds on here and all the time ill ask somone if I can use one of there backgrounds to turn into a layout to put on my site for people to use and ive always asked for permission but there are some people who havent signed on since like 2004 and I dont think they will but I really want to use some of the things they made but I cant get permission cause they never see the request. So when that happens can I just take them? Will it be ok if I put a note on my site that says like "if you were the designer of any of the following images and you have not been credited or would like your image removed feel free to contact me."

Thanks for the help!