what is the best editing photo software for beginners on imac

The built in editing capability of Photos is very good. Since it keeps the edits in a database and doesn't touch the original files it's nondestructive in nature like many of the high end digital assets managements apps.

No. The Retouch tool will let you remove very small objects against a plain background:

However, the Clone tool in Photoshop Elements and Gimp can do this:

The best way to find out is to take a test shot and see what you can do with the Retouch tool in Photos.

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I was curious as well; will the built in editing software allow me to remove objects (chairs, trees etc)?