While a statement asking people not to use your 
images with or without your permission is all 
very well, here are more effective measures:

(1) Watermarking
When editing & resizing scans, slap on a 
watermark.  Urls, emails, site names or other txt 
are often used.  Bear in mind that some people 
will still steal them and put them on their 
site.  Some may edit the watermark out, fade it, 
slap their own mark over it... so use with the 
following measures.

(2) Transparent Gif
http://futuredbztrunks.homestead.com/celgallindex.
html
Just go to my page & save the picture of the poop 
on the top right & you will in actual fact get a 
transparent gif.  Just place it over your cel 
image.  Depending on how you make you pages you 
should be able to resize it to fit.  Should be 
sufficient to place just over large images.  Or 
if you have several images on one page then just 
stretch it over the whole page to save time.

(3) Disable right click
Get the html script here 
http://www.java-scripts.net/scripts.phtml?
cid=9&sid=57
Just copy it & paste it on your page.

Of course some people have the know how to get 
round all these but it does deter the less 
determined.  But when all the measures are used 
it should have some significant effect!
Please add any more that people know off.