Is it illegal to edit a photo you found on google?

Depends on the photo, really. Often the easiest way to find out whether or not a photo is protected is to check the very bottom of the webpage where you found it. It may say something like “All content property of…” or “Copyright… All Rights Reserved.”
Sites that often host artistic images like Flickr.com or DeviantArt.com will usually have the specific rules attached to that particular photo by the artist.
Truthfully, people do it all the time. As long as you don’t intend to sell the image, reproduce or claim it as your own art.. you’ll usually be fine. But it is best to look for the fine print. It really depends on the individual image each time. There are different rules for different images all over the internet.

If it is just for your personal use, you are free to edit it as you please. Fepending on what you do to it in editing, who owns rights to it, and what rights they are, it is likely not legal to post it publicly or ro use it for resale as your own work.

Anything that is published by someone (and that includes published as a website) is automatically covered by copyright protection when it is published. The content owner can give up some or all rights if they so desire by stating so in a copyright statement. The GNU General Public License is often used by content generators who want their work usable within certain limits. Other such limited rights licenses exist, and of course, a content owner is free to write their own. A photographer for a certain religion might ask that their work only be used in non-profit projects supporting that religion. They could specifically exclude any right to edit the work in any way which they, in their sole judgment, feel denigrates the cause they support.