There is no need to be concerned about this. Put the heaters on walls where your pictures won’t go, or clearly mark where the heater is so that you always remember not to put pictures in that spot. The heater is a panel design, and will never take up more than a 1m2 area per panel. You can also paint the baseboards and lower-half of the walls where pictures don’t go. As well as, paint the ceiling or floor instead.
Lastly, you could even put the heaters in-between pictures. However the best way is to paint low on the wall. There is no risk, and this is already done for standard convection radiators.