Chose the picture file that you want to edit for your blog header. There are several different edits you can make to the photo and I usually just use the "auto adjust" to get the color/contrast better. In this case, because my file size is larger that I want it, I used Re-Size(at the bottom of the list) first in order to get it the size I want it.
After you have the picture picked out, the first thing you need to do is re-size the image to fit the blog header. There's not really a standard size for the header but I have found through trial and error that the size that seems to work well is 1100 x (350 to 400). In order to resize the photo to this size, I have been using PicMonkey , a free online program to modify pictures.
Within Photoshop for Design Header Image
Within GIMP to Design Header Image
