The above photos were tweeted out by a Michael Moore (not the director), with the comment:
The media constantly brought up Trump’s tanning.
Is anyone going to ask about Biden’s new head?
Moore is probably just making a snarky comment about Biden's hair transplant - a topic that has actually be discussed in the media for decades, here's an example from 2008 dating the story back to 1987, and including another photo where he does not look exactly the same:
It is a fraying, combed-back helmet that barely covers a longtime fact of Washington life: The senator from Delaware has taken steps to pre-empt baldness.
The most common hypothesis is that he received a hair transplant, where follicles from the bushier back of the head are grafted onto fading spots closer to the front of the dome.
In 1987, a Washington Post reporter asked him to confirm the theory. "Guess," he responded. "I've got to keep some mystery in my life."
Snark aside, in the more Q-like corners of the internet a frequently popular theory is that various people are imposters, clones, or body doubles. Evidence for this theory comes from poor-quality and misleading photo comparisons like Moore's. So his popular tweet was an instant hit in those circles, with a significant number of those commenting raising the possibility of the photos being of different people.
The two photos come from two inaugurations, Jan 20th on 2009 and Jan 20th 2021, 12 years apart. So some differences are to be expected. The photos are most easily distinguished by the color of Jill Biden's dress or coat: red in 2009, grey in 2013, blue in 2021.
The "Old" photo Moore picked was from 2009
The "new" one is actually from 2013:
Biden's in 2021
Notice in 2021 there was again direct sunlight, so Biden is squinting, leading to a more similar photo to the 2009 image. So a more honest comparison of how Biden has changed (and over a much longer version) is to compare 2009 to 2021