For safety reasons they are supposed to be scewed to the wall, you can just place them against the wall but depends on what type of household you have.
If its just adults then maybe go for it, but if you have pets/children who might knock it them then maybe not.
If its just for cosmetic reasons why not just get the radiator changed? They are quite cheap these days.
Someone with more knowledge may be along soon, just saying this from personal experience.