Moldings are incredibly easy to make. Just use layer blending effects. A little Bevel & Emboss goes a long way.
Assuming you're using Photoshop:
1. Create a new layer above your wall layer(s).
2. Draw a rectangular marquee across the bottom of the canvas, to outline what will be your base molding. Fill with white, or whatever color, wood pattern, etc., you want.
3. Double-click on the layer's thumbnail to bring up the Blending Options dialog. In the Bevel & Emboss section, apply a bevel. Play with the settings to learn what they all do.
4. Draw another marquee across the bottom of the canvas, but this time make it much less tall. This will be for the strip of quarter-round that is almost always found along the bottom of a base molding.
5. Press ctrl-J to copy the selection to a new layer. Tweak the bevel to taste.
6. To add more detail, just keep repeating steps 4 and 5, to form the various parts of the molding. You'll find that judicious use of additional blending options, such as Drop Shadow and Inner Shadow, will add much needed realism to various parts.
7. If you want a crown moulding, just repeat the above process at the top of the canvas instead of the bottom. Don't foget to include a drop shadow on the molding's first layer, since crown molding always casts a shadow onto a wall.
If this whole process takes you more than a minute or two, you're doing it slowly. Have fun.
ETA: If you want to save a few steps, get good with drawing custom contour curves for your bevels. You can create a complex molding in just a couple of clicks if you've already created the right curve for it.
)