Hi there, I wanna to use 2D textures in my game(3D). I am designing my textures in Photoshop. I have a question. What is the main difference between Image Size(width & height) and Resolution?1 What is the best way to use this texture (run fast,use low texture memory,...) : 1) a texture with 300 resolution and 128*128(dimension) 2) a texture with 72 resolution and 256*256(dimension) Somebody have any idea ?

You're getting yourself confused in your terminology.

The first number you presented (what you called 300 resolution and 72 resolution) are the DPI resolution, or the dots per inch resolution. It doesn't actually effect the image file size, it affects the image display size. A normal monitor displays at 72 dpi. Normally JPEGS and PNGS default to this. If you create a canvas for a 4" x 5" picture, it will default to using 72 dpi, or 72 dots per inch. That exact same image could be printed at 144 dpi and it would come out half the size. The dpi is set for an image to reflect what medium it is going to be diplayed with.

Your second number, 128x128 or 256x256, are your actual resolutions, or pixel counts. It is a more literal interpretatioon of an image. This is image is 128 x 128 pixels in size. This determines the actual file fize and how quickly an image will be rendered.

Creating textures you should use 72dpi.
300dpi is usually used for printed media (combined with CMYK colors).