You could also try using the texture coordinates to have wow crop the texture for you and not have to photoshop an entirely new texture by putting the below code in your texture code of your xml.
Code:
<TexCoords left="0" right="1" top="0" bottom="1"/>
Been awhile since I've used this but if memory servers me the values go by the textures dimensions as a percentile. For example left="0" (0%) and right="1" (100%) so if your image was 100 pixels wide and the part you wanted was from the far left to the middle you would do left="0" and right="0.5".
Basically you take the image and open it in a photo editing program like photoshop to figure out the pixel dimensions of what you want and the texture itself. Then if what you want for example starts 5 pixels from the left you would take 5 and divide by the full textures width (say 100pixels as the example) so your left starting edge would be left="0.05", just repeat that for the other 3 sides and you should get what you need.
Its abit more work but saves having to make a texture for something thats already loaded for use. You may end up just having to make a texture anyways since the quest log is probably like the character frame in that its seperated into 4 large textures so you arent going to get the full size of what you want and it may look bad if its stretched too much.