Once you've sculpted your beautiful object, you may want to be able to texture it. The brilliant things about sculpties, is that unlike prims where you either have to recreate the object outside of SL or just have a really good idea of how things will map and hope you're right, you can paint directly onto your object in Blender. It's sex.
Once you've done your mapping and your sculpting, go to Texture Paint mode in the pane with your 3d object (assuming you work in the split 3d View/UV Image Editor with the bottom Buttons Window workspace that I find most useful). In the dropdown next to that, there's the view modes: choose Textured with the odd little nubby thing at the top, so you can see what's going on in your object. Select the entire thing with A, and in the texture view pane, create a new image. This one should be big! Bare minimum of 256x256- I generally work in 1024x1024 with the idea that I will scale it down to 512. I want it to be at least double size of the final product though, because I'm not so good at fine control, and really, shrinking down is always a good thing.
Warning: if you have only one face selected when you add this image, only the painting on that face will show! The rest just won't do anything! This can be useful when you, say, want to work with a cluster of faces and keep them separate from the rest of the object. This is not useful when you accidentally do it and can't figure out why the hell your painting isn't working except for one little square.
Unselect in the 3d view with A again, and your object should now be black, black as my soul, with grey lines denoting the faces. Your image should be similarly black, although without the grid. You can, at this point, just start doodling with the default paintbrush, which is rather large, and set to white. However! Hit F9 to be taken to the Object tool panel at the bottom, and there is a panel there, Paint, that gives you control over your brush! Opacity, mode, size, colour, etc, as well as a couple of different painting modes. Brush on your highlights and shadows! It's magic, not only does it show on your 3d model while you're painting, it also shows you exactly what section of the texture you're painting on. I still find I lack fine control, so I tend to do large markers for myself to use in Photoshop, and save off a number of layers (for instance, I do one image for shadows, another for highlights, in the black/white, and use Channels to pull out the stuff I want). Any time you want to use a new image, save your image off with a useful name, and create a new one in the Image menu of the texture pane again.
Another fun and easy cheat for basic shadows, is Ambient Occlusion. This will light your model with global lighting. Its results are very noisy and gritty, so you may want to do a bit of post processing, but it still gives you a quick and easy baseline that's great for some basic shadowing to start from.
Go back to UV Face Select mode, select all (A), and create a new image again. This also needs to be big really, again bare minimum of twice size (I generally use twice size, and then do a lot of smudging/blurring in PS to tone down the dirtiness). With all still selected, hit Ctrl-Alt-B, and choose Ambient Occlusion from the dropdown. It takes a bit to do all of its interpolation, and you can watch the image being build up in rows. If you've still got Textured selected on your 3d model, you can see exactly how the shades of grey are mapping to your model. Save that image off for use.
It is fully possible to do start to finish texturing in Blender- however I find the tools in PS are easier for me to use, due to familiarity, the vast toolset available there, and the fact I'm not having to worry about rotating a 3d object to get around it in the middle of what I'm doing. There are also a number of other texturing options available- setting up good lighting and doing a Full Render instead of the Ambient Occlusion, for instance, will get you more real baked lighting. Materials let you set things to have inherent reflectiveness for the lighting to play off of as well, so a really well textured shininess is not that difficult. But, these are things that have so far been beyond me- the lighting never seems to listen to me at all, and I ended up with crinkly cellophane looking highlights the time I tried to do a full render. I'm obviously missing a step somewhere, that I'll figure out eventually, once I decide I want something that I can't get otherwise.
You can also open images in the same menu where you create new ones, to do some of this stuff to an image you've already partially prepped (with, say, a pattern), as well as seeing how your current image will map once you've got all of your layers sorted (and paint directly on that, as well- with brush settings you can conceivably paint on some of your detail there).