Here is a tutorial on how to use the path tool in Photoshop to create a smooth flowing line.

I use this technique in web design, which enables me to create a layout with a corner image with a unique touch to make the image seem more in tune with the layout.

Firstly, load your image into Photoshop. I've chosen this picture of Kate Beckinsale. Here is the original photo, and then the finished image.

First Step:
Select the pen tool. This is found on the left icon menu, and looks like a pen nib. Now if you want an image that sits on the left hand top corner, you need to first make a point on the left hand edge of the image, near the bottom. The closer to the edge the better, as you won't lose any of the image that way. Make your second point on the top edge on the right. Once you've done that, you should have a line cutting across the image. If you click somewhere on the line (in the middle-ish), and drag, you'll notice the line curving. Once you have a curve you like the look of, and you've let go of the mouse button, you'll notice something that looks like this...

Second Step:
Now if you want to add more than one curve point, click and drag on the line as before, and you'll have something that looks like this...

Third Step:
Now we have to close the path. To do this, you need to cover the part of the image that you want to appear on the final product. So in this case, you'll be clicking in the very top left hand corner. You'll notice that Kate's head is covered in your foreground colour. Now click on the point you first started on in the bottom left (you'll notice a small circle appear by the pen cursor which means you're completely closing the path. If you have a history menu on the right, it'll say Close Path. You'll now have something that looks like this...

Fourth Step:
On the right, you should have a menu which has the layers on it. Click and drag the background layer onto the create new layer button. This will duplicate the background layer. The icon is indicated here...

Fifth Step:
Now hold down the control button and click on the top layer. This will select the shape covering Kate's face. Go to the menu Select-->Inverse. Now make sure that on the layer menu to the right that the paintbrush icon is on the middle layer, the duplicated background layer.

Sixth Step:
Cut the pixels (hold control down and press the x key). Check that the layer is now only half filled with image. It should look something like this...

The image will still look the same because of the background (bottom layer). So to make it look a bit better, you can now delete the top layer, with the coloured shape by dragging it onto the trash can icon underneath...

Seventh Step:
It will look exactly like the first image you loaded into photoshop, but then click on the background layer (bottom layer) and go to the menu option Edit--> Fill, and then choose a colour for the background colour. I've chosen a soft green to match the leaves. If you've already chosen the colour and you have it on your palette to the left, make sure you select forecolour (if it's on the square to the front) or background colour if your chosen colour is on the square to the back like mine was...

Eighth Step:
Now as you can see your image is complete... almost. Now you can add effects to the photo layer by selecting the layer and clicking on the Add a layer style icon.

I'm going to add an emboss effect and a drop shadow to the image. You only have to choose one though, and from that you can choose any effect you like. And there you get the finished image!