Here are some helpful URLS if you are new to OL and not terribly familiar with SL.... at least I hope they will be helpful.
Textures are images used to bring visual information into a Grid (SL OL...)
Most textures on the web are copyrighted images and are not just to be grabbed. Some textures have been freely given for all to use. Most of these textures have been created by folks who ask for a link back to the page where you got the texture. Using copyrighted textures without permission is stealing.
I learned how to create textures (still learning actually) by using GIMP -- an acronym for Gnu Image Manipulation Program. It is an open source image manipulation program similar to Photoshop but the code is open source. (If possible try not to think of Open Source as FREE.... donations support the efforts of the folks who work to grant us all a more level playing field.) Gimp can be downloaded from http://www.gimp.org/
The term TILING refers to the repetition of a texture across a face or plane.
The term SEAMLESS refers to the undetectable repetition of a textures across a face or plane.
512 by 512 is the standard size of images used in OL. If you do not convert your images to this size before importing them into OL, your images will be converted into even sided rectangles with dimensions of 128,256, 512, or 1024. The higher the number the more information the image contains and the more information that has to be stored and the more demand is placed on the servers and the more likely lag is to develop.
Psd format (photoshop format that can be read by Gimp) is one of the main formats for images.
OL uses jpg and tga images. Targa (tga) images can be tricky as these include an alpha channel which is interpreted as transparent. Halos can be seen through alpha textures unless the alpha channel is processed just right. This is beyond the scope of the basic info I'm trying to include here.
Alpha or transparencies do not stack well in OL. If you layer one prim with an alpha and put another alpha prim next to it you might not see some of the texture on one or both. Example: Prim hair that is textured with alphas will become invisible when anyone looks at your avatar if the background behind your avi contains an alpha.
Hyper Girl has a bunch of info on where to get templates for making hair, clothes and the like.
Many of Natalia Zelmanov's Mermaid Diaries tutorial entries on how to use textures and make things can be directly applied to OL even though she wrote these for SL.
Some free skins in psd can be found at: http://www.vintfalken.com/free-full-perm-female-skin-textures/
That is all I can think of now. Will add more as I can.