I guess the question pretty much explains its self.
However, if you need more clarification don't hesitate to say so. 🙂
I find the easiest way to move wall hangings up and down is by pressing the ; and ' keys. Holding down the mouse until it finally changes to an up/down arrow is imprecise and finicky. The keystrokes are a lot easier; I can even get them to work in live mode. That said, there are numerous wall hangings that can only be moved up/down with mods, the skill certificates for example. Here's a link to all the made-shiftable mods that I use: http://www.moreawesomethanyou.com/smf/index.php/to...
If you need to move regular objects (not wall hangings) up and down, you need the OMSP for that: http://modthesims.info/d/428179
If you mean pictures and other things that go on the walls, if you place them, then click and hold the cursor on the object while moving your mouse up and down, you should be able to adjust the height. I've not gotten it to work while in "live" mode, only while in Buy mode (in other words, I couldn't adjust the paintings my Sims had just completed).