glad your back and at least feeling somewhat better. 

for your artwork if you post it you could include a watermark like a faint but visible symbol in the center.  im not familiar with the rate or risk chance of art theft but ive gotten the impression that it is more rare than not.
for the characters make it clear they are yours to anyone who views the image.  cant think of much that can be done on this site to prevent theft.  aside from reporting and pursuing the thief if it happens.