We are not allowed to give legal advice. In many countries only a lawyer can offer legal advice, and lay advice is illegal. I can suggest to you that for tax issues you might need to talk to an accountant, any lawyer will tell you that, and send their bill.
So: Yes, you are allowed to upload pictures. You are even allowed to sell the pictures via a platform like Adobe stock. But you need to follow the local tax laws and the local busines laws (normally you will not be considered as a business, so that part will be easy). And for that you will need someone who knows those laws.
In general, Adobe asks you to fill out American tax formalities, which state what taxes you need to pay in America. As Belgium has for sure a tax agreement with the US, you will need to do what you need to prove that your tax layability is in Belgium and that Adobe is allowed to release the money you make with 0% American tax.
You will be responsible for paying the local taxes.
For the details, however, you need to rely to some legal advisor. Either find an association providing basic advise for free or a flat fee or hire a tax laywer/accountant to advise you punctually for this problem.
Regards, Abambo Hard- and Software Engineer and Photographer.