6
Editing the style.css would not allow you to specify an image per block, only one image to be used with all blocks. If you want to do something in a theme dependent way, but still have some flexibility (I assume you want a different image per block) I'd suggest, using the default theme as an example, to change theme_blockleft.html and them_blockright.html to read:
<div class="blockTitle"><img src="<{$xoops_imageurl}>/<{$block.title|replace:" ":"_"}>_image.jpg"><{$block.title}>div>
<div class="blockContent"><{$block.content}>div>
Then you'd place all images in the directory of the theme you are using and name them blocktitle_image.jpg, where you would replace blocktitle with the title of the block, replacing spaces with an _.
Be advised however that every block you now display should now have a corresponding image or a broken image link will be displayed.
There is a possiblilty to include PHP code into the template to check wether an image exists but this would become so cumbersome you'd be better off adding this block icon functionality to the XOOPS core.