All the images in Wiki Commons are free to use legally...it's only about attribution.
The ones under Public Domain don't require you to attribute to the source to legally use the image.
You can search any image by keyword.
Just click on the image you want to use,
Click on 'Use this image on web' ( you'll find it either above or on the right side of the image)
Now, You can see on screen all the information you've to use to properly link to the source of the image.
Just fill the required information while using the image and there'll be no legal hassle.
You'll note that the images under public domain say 'Attribution not legally required'...but I personally do attribute.
Hope it helps.