At the end of the day, if your superiors tell you that you have to do something, there is no way to politely decline to do it, and any refusal may be grounds for dismissal or being stuck on the slow train to nowhere with your career in the office.
To minimize these sorts of requests, I would recommend:
- Explaining the tradeoff from putting other tasks on hold
- Teaching them how to conduct the tasks themselves
- Recruiting a minion you can hand these sorts of tasks off to
No Silver Bullet
These are all long-term strategies that will require effort. They are also not mutually exclusive strategies. A combination of all three will probably work the best, as there are all sorts of different types of people who ask for help, and some methods may work better for some than others.
As stated in the executive summary, at the end of the day if the boss tells you that it's your job, you are better off doing it. So if you want to avoid being asked, it is better to start sooner rather than later to start reducing the number of requests.
Next time a superior comes over and says, "Hey, could you edit this photo?" you can gently remind them that taking your time to do this task takes your time away from doing a separate task, for instance:
Sure boss, I can edit the photo. Today I am setting up a computer for Alice since she can't work until that's fixed, and recovering some files from a backup so that Bob can finish his work for a deadline tomorrow. Where would you like me to stick it in?
Assuming you are truly busy with more important tasks, a rational boss will likely say, "Oh, never mind, I can handle it." Regardless, it sends the clear message that you have other work that is also important, and while the boss can make you do it, it will be at the expense of other employees (not just your time).
Teach a Man to Fish
While I was writing this, jmort beat me to the punch so I will just quote from what he so eloquently said:
next time someone comes in and asks you to change the header/footer, don't do it at your desk; instead, walk with them back to their desk. Have them sit down at their computer, and then walk them through the steps. Different people learn differently, some visually, some auditory, and some by doing, so if the person you've walked through it keeps coming back, maybe try doing it yourself while they watch (visual) and then have them repeat the steps back to you (auditory).
There may be interns, 'administrative assistants', receptionists, or other folks who don't have enough real work to keep them busy. They may also be more than happy to do these sorts of tasks if you ask properly. If you can find one that can do the job quickly and competently when you ask, you can hand off the tasks, and eventually just tell your boss, "Christine is really good at this sort of work. Do you mind if she does it instead?" and hopefully create a direct route for your bosses to ask Christine rather than asking you.