With html 4.01 and .css file, impossible to display other format than .PNG

[FRENKEL GUY]

I've tried to display .jpg files or .GIF but it was impossible. Maybe there's a problem of DOCTYPE declaration?
I used HMTL 4.02 (Front Page).
Thank you for your help, i'm completly loosed and i'm also a begginer in site construction.

[Rabbit]

It's hard to say what's wrong without seeing the code.