Is it illegal to post pictures from google if you collaged these photos in photoshop or gimp and put some of your own photos in it making it a huge wallpaper that youre going to post in a blog? is that illegal?
As long as you’re not making money off of it, you should be fine.
Even if you do end up using someone’s copyrighted content, you could easily argue fair use. Whatever the case, (and don’t take this the wrong way) your blog will probably never have enough traffic for these copyright holders to ever see it.
if there is any copyright claims on the picture you’re using it would be illegal as long as you are not claiming any ownership. Put a disclaimer on it if you must.