Terms of service are long, written to be unreadable, and universally accepted without reading. A handful of provisions appear in nearly all of them and are worth knowing.
The licence you grant
Uploading content typically grants the service a broad licence to use, store, modify and distribute it. That is necessary for the service to function – it cannot show your photo without a licence to display it.
The question is scope. Some are limited to operating the service; others extend to marketing, or to training systems. The difference matters and it is usually one sentence.
Unilateral changes
Almost every agreement lets the company change the terms and treats continued use as acceptance. Effectively the agreement can become something you did not agree to.
Arbitration and class action waivers
Common in some jurisdictions. Disputes go to arbitration rather than court, and you cannot join a collective action. This removes the main practical remedy when a company harms many people slightly.
Some agreements allow opting out of this within a short window after signing up. Almost nobody does, because almost nobody knows.
Termination
Most services can terminate your account at their discretion. If your account holds purchases, work, or a business, that is a substantial exposure.
What to actually check
Search the document for the words licence, terminate, arbitration, and change. Read those paragraphs. That takes three minutes and covers what matters.
The realistic position
You have no negotiating power and the alternative to accepting is not using the service. Knowing what you accepted is still better than not.