In some cases giving credit to the picture´s owner is a good idea. It´s good to host the picture you are using yourself somehow (example:
www.uploadit.org... ) - "Hotlinking" might not always be a good idea.
You could stop off at Zedd's place for handy tips on this
and, well, just about everything else too.
Also, take a peek here for some Image Size Guidelines . And remember when trying out
new things (tags etc.) that you can always make use of the 'Preview Post' button to ensure all formatting is correct before you submit anything.