pretty much, but only for those modules that you can edit (this is for example not the case for the Recent Topics or the News module). Edit your module in the Portal Structure site and place the following codes:
<div class="block1" width="100%" height="100%"> at the beginning of the module, and
</div> at the very end of the module so that the whole content is wrapped.
The
red part is the class of that modules background. All div's with the same class will get the same background. That means if you want another module to have another background, name it's class "block2", "block3" and so on. Note that a class should always start with a letter.
Now you can go to your CSS ("Styles / Colors / CSS Stylesheet") and enter these codes to the textfield - one for each class:
.module
.block1 {background-image:url("IMAGE URL 1")}
.module
.block2 {background-image:url("IMAGE URL 2")}
etc...
