New background & header / why owls? And HELP!

As you know, I've been wondering for a while how to do cute stuff on Teachers Pay Teachers.  I've also been longing for AGES to learn how to do customized blog backgrounds and headers, rather than using a premade one or paying someone.

I figured out how to do both, and they are pretty similar.  I designed this header using powerpoint.  I got the both the background and the owl from Teacher Pay Teacher (in these stores:  owl (From The3amTeacher), background).  Then I uploaded them into powerpoint and layered them according to that tutorial I posted the other day. 

Now, why owls?  Not to be trendy, because owls seem to be EVERYWHERE nowadays.  But it stems from my love of all things Harry Potter.  I love Hedgwig and was devestated when she died (yes, I know she was a fictional animal character, but still!)  I also like that they are the symbol of intelligence and wisdom.  As a teacher, I hope that I too can be seen that way.  Plus, it's always fun to say you "give a hoot about reading".

I also love pink and I'm not afraid to admit it.  I like hot pink.  It must be the "child of the 80's" coming out.  I'm sorry, but I do.  I hope that it doesn't turn you off and away from the blog.

Now for the HELP part:  I love this header, but I can't get it to fit.  How do I resize it?  I think its too fat but and not wide enough.  I also want to change the background, but I can't get my image to upload.  I'm super confused and would just love some help.