DPI and Screen Size
The relationship between image size in pixels and DPI (dot per inch) or resolution as it's labelled in Photoshop is one that seems to confuse many newcomers. The first point I'd like to address is the peculiar notion that web images must be 72 DPI. This is just nonsense: the size of an image on your screen will be completely determined by the number of pixels on each side.
This image is at 240 DPI |
This image is at 72 DPI |
Not a lot of difference, is there? 
The file size is the same too - down to the last byte.