I don't ever just try to compare pictures. I find it useless. The city just did that when asking people if they have ash trees due to an outbreak of emerald ash borers and a group of pictures was not half as useful as the various websites for that purpose. The tree ID websites are a general leaf shape, whether it's one leaf per stem or several, whether the leaf tips are pointed or rounded, whether the branches or multiple leaf stems grow across from each other or alternate.... You aren't trying to compare an exact picture. You line up general tree characteristics and most are divided by parts of the US or growing zones and many states each have their own website to help narrow it down more. Sometimes it takes a bit of clicking on more than one option if you don't know the terms used but most of the time after looking at the results it's fairly obvious which one applies. When it comes to a few very similar species that have the same leaf shape, branching, bark, etc... then I just post pics to the garden forums and let them figure it out. Although frequently I don't know or remember what the tree looks like in the opposite season so if it comes down to flowers or seeds to ID it exactly I'd have to wait 6 months but it's rare to need that exact of ID to rule out any remaining toxic species from safe ones.