I have just bought a house and there are grey spots on the painted walls, particularly where pictures have been hung. can anyone tell me what they are likely to be and if any special preparation is needed before repainting?
Thanks

It could be spots of mould, these can be caused when cold air is trapped behind a picture creating a small amount of condesation on the wall. Just give it a good clean before painting.

If it is the fact the wall is so cold it may well be worth seeing if you can insulate it .

If this is an inside wall, then bear in mind, I'm only a chippy

If you mean 'patches' as opposed to 'spots' and the shape roughly corresponds to the shape of the picture, it is just that that patch has not seen daylight for a long time and has not faded like the rest of the room.

As sparky says, clean with sugar soap before painting.

But I'm only a decorator