Resolution (dpi) doesn't have any meaning for
pictures displayed on a screen. (We usualy
consider that a screen is around 72-75 dbi
but that doesn't mean anything).

Screen have a fixed number of pixels. Most
people use a size of 800x600 or 1024x786 pixels.

So if you want that your pictures fit on a
screen, you probably don't want them to be
bigger than 500-600 pixels. (whatever the
dpi : you can put 1 dpi or 10000, it won't
change anything :)

If your picture doesn't have the right size
ratio when displayed in your webpage, it's
probably because you have wrongly set the
"heigh" and "width" tags in the HTML code.
And so the web browser has to modify the
picture to fit the specified size.

Cheers,
Yann Stettler