Hi Ayesha!
As I mentioned before, I wouldn't recommend buying BodyPaint if you are only going to use it for SL. It's pretty expensive for that. However, if you have other uses for it to, it can be well worth the money.
One of the problems with writing tutorials for BodyPaint is that the interface is so completely customizable that things are likely to look different for different users. So I'm going to use one of the default layouts. To make your screen look like this, go to the main Menu bar, to Window > Layout > BP 3D Paint (as shown
here.)
To import a Photoshop image, assuming that you have already made the model "paintable"1. There is a Materials palette in the lower right corner of your screen, showing the three materials that go on the Avatar. Double-click the sphere icon, to the left of the name for the material you want to apply a texture to. This will open the Material panel.
2. Click the Image button in the Material panel to open a browser that lets you find and load an image. BodyPaint can read lots of different file formats, including .psd, so just find the one you want. (Note that it can't read some of the adjustment layers, or some blending modes (such as Offset,) So work on a copy of the file, not the original, if you are using them.
3. A dialog will open telling you that the image isn't in the Search Path, and asking if you want to make a copy. Unless you really want extra copies of your documents piling up in the folder where you keep your mannequins, tell it no.
That's it. It will take a moment for the program to read in the texture, but then you're ready to start painting.
To save the texture from BodyPaint as a .psd file1. Right click on the texture Thumbnail (from any layer, it doesn't matter.)
2. Choose Texture > Save Texture (or Save Texture As...) from the fly out menu.
3. Save the texture, making sure you are using the file format you want.
To reload the image, after tweaking in Photoshop1. Double-click on the Sphere Icon, to open the Material panel again.
2. Click on the flippy after the text field with the Image path.
3. Choose Reload Image from the fly out menu.
That's really all there is to that. The important thing to remember (so I'm going to repeat it,) is that BodyPaint won't read all the various special Photoshop layers, such as adjustment layers, clipped layers, Smart Objects, and some Blending modes, the way that PS does.
If you overwrite a file that has those things after opening it in BodyPaint, you will lose them permanently. So work on a copy of the file, not the original. (You might want to delete, merge, rasterize or apply such things on the image you're taking into BP, so you can see what your texture actually looks like. Sometimes BP's attempts to read things it can't read aren't pretty. Otherwise, just hide those layers in BP.)
For BP tutorials, you'll find a few
here, or just Google for them. But there's not much in English, I'm afraid.
Hope this helps!