Hi jodiescreations -
First of all is the logo too big or too small? Right click on it and then click Properties and look for the pixel width and height. Your logo should be 270 px wide by 100 px high to look perfect in the header.
If your logo is less than this, you are going to want to add a nice color background to the logo using a photoediting program.
If it is larger, upload it to your Auctiva account and then when you are on the manage images page, put an check in the checkbox and then click the edit button. You will see the width and height boxes under your image - enter 270 in the width and hit the Tab button. Height should change automatically. You may have to crop the image a bit to make the width and height close to what you need.
Hope that helps!
Cara