512x384 will be scaled up to 512x512. Textures in blitz must be of a square power (64x64,256x256,512x512). They don't have to be square tho, they can be like 512x256. Just as long as the numbers are powers of 2.
Also, when blitz scales texture, like yours, it doesn't make a very good job of it :)
Also, when blitz scales texture, like yours, it doesn't make a very good job of it :)