You have several questions. The first is "Should I act?", and, if yes, "To whom?" and "How?".
Firstly, the very last option you gave is your first go-to answer:
Ignore it and mind my own business?
When in doubt, it ok to wait and try to figure out if it really is a problem or not.
For me, I determine if it's a problem for me or the business. The only clues you give regarding how it affects you are the following:
I do find this to be somewhat unprofessional ... the objectification of women which is distasteful and disrespectful.
So does it harm you or the business? If you have clients, customers, or investors coming through then yes, it's clearly a business issue. If you've heard or seen negative reactions from your female co-workers that suggest this is affecting them, their work, or making their work environment worse, then it's a business issue.
You probably aren't in a good position, otherwise, to determine objectively if the displayed images are truly denigrating. You have an opinion on the matter, certainly, but it may be that everyone else in your office has a different opinion where this isn't a problem. So I don't think you can generalize - in the absence of walk-throughs of the office area for other business professionals outside the company, you may not have much objective reason to suggest that he shouldn't have those images on his screen.
There is one possible resource, though - does the company have a dress code? It's reasonable to suggest that any displayed images within the office environment roughly meet the company guidelines for the same reason they apply to oneself. If one of your co-workers chose to come into the office and work in their underwear and it violates office policy, it's reasonable to suggest that similar images should violate office policy.
Beyond that, if you aren't personally affected, and you don't have a good basis or foundation to suggest they change their wallpaper, then you should probably ignore it.
Assuming you've decided to pursue this, and see it as important to the business or yourself, then how you approach it depends on the foundation for your claim.
In general, first approach the co-worker. Tell them what you've observed, and why you believe it's inappropriate. Indicate that if they disagree, then you'd like to go with them to an appropriate authority within the company to help discern the best choice for the company.
The most important thing is to avoid a sense of judgement. It's not that the activity/images/interest are wrong, but that the time and place require a high standard for whatever reason you've founded your claim upon. Make it about the images, time, and place, and not about them or their preferences.
Being able to be objective and using company or industry manuals or stated standards is important here.
If it's just a personal distaste and distraction, mention it to them, again without judgement. Use "This makes me feel..." and "I am uncomfortable..." statements which reflect your feelings, rather than "That content is ..." statements which subjectively judge the content.