Is it possible to import textures from your computer?

I have seen many textures while exploring second life that look really cool and I was wondering... if it was possible to import custom textures? It would be great for signs, banners, etc.

Sounds like someone skipped orientation.

Yes., you can upload textures from your computer to SL. In fact, that's how virtually every single texture you see in SL got there in the first place. With very few exceptions, everything in the SL world is user-created, and that includes almost all the textures. You'll find a handful of items in your library that were made by Linden Lab, but literally everything else in the world was made by people just like you. Individuals in SL upload tons of new textures every single day.

To upload a texture yourself, simply click on the File menu at the top of the screen in SL, and then click Upload Image. It will cost you L$10 for every file you upload. Supported formats are TGA, BMP, JPEG, and PNG. For more info formats, and for a list of usable image sizes, see the sticky at the top of the forum.