Generally speaking it’s best to fit radiators beneath the windows of a room. The reason for this is two fold.
Firstly, cold air falling by the window will be balanced out to some extent by the warm air rising from the radiator. Fitting them on the opposite wall can actually cause a noticeable draught, since warm air rising on one side of the room and cold air falling at the other exaggerates the flow around the room.
Secondly, and purely a practical consideration, is that for the most part, the wall area beneath a window doesn’t tend to have furniture in front of it. You therefore make use of unused wall space and, at the same time, do not end up shielding the radiator with a chair or the like.
If you have double glazed windows, the air will not fall by the windows to anything like the same extent. Therefore you have more flexibilty in positioning the radiators. This can be a big help especially if you wish to have full length curtains which would cover the radiators if they were under the window.
On a similar point, avoid placing objects which will reduce the effectiveness of the radiators. Shelves positioned immediately above can have an adverse effect so make sure they are at least a couple of inches up from it. If a shelf is set in this position and is reasonably narrow, the air will simply flow around it. Radiator covers without enough allowance for air circulation can also be a problem. Make sure air can pass freely in at the base, and out through the front at the top.