Because there are so many licensing and fair-use issues out there, most legit, mainstream blogs (newspaper blogs, for example) avoid using third party images. Instead, they stick to photos they took themselves, or use images from royalty-free sites or services they subscribe to (AP, Reuters, etc.).
I'd recommend using your own or sticking to a royalty-free site, unless you have permission from the photographer/owner to use a particular image.
As far as CartoonStock, it looks like you must pay to use their images, according to their FAQ.
http://www.cartoonstock.com/faq.asp#2
Either way, you'll want to credit the source/owner -- as you've outlined would work, and double check with the site that the images you are using are in fact in the public domain and legal to use. Royalty-free sites for cartoons/comics are scarce, but you could try:
You'll have more luck finding images:
T
hen there are sites that have images falling under creative commons, which may be used with proper attribution. Just be sure to follow the instructions and credit accordingly.
Want more. Here's a list of 100 free stock image sites:
