Rebecca left a sweet comment on my
photo post about liking my headers and wanting to know more about software. I responded to her comment, but I thought it would be fun to go through my process for the headers using Picnik, since I think that is just about all anyone needs. For around 25/year for Premium and regular updates to it I think it is a great deal.
Step 1: Find a photo I like. (this one is my purple flowering kale in the garden) It needs to have enough interesting detail in a narrow band for a header. Rotate it until you get the right angle.

I rotated this one left, once.
Step 2: Crop it. I know the general dimensions that look good and fit, I try to have a focal point, and I like to follow the
rule of threes. Ever wonder why your camera has that nifty grid to divide the space into 9 parts, yeah, that would be it. I use it for balancing positive and negative space as well. Sometimes I just go for content, but the ones I like the best follow those rules. (left two thirds positive space, right third, negative space)
Step 3: Adjust exposure (this one was over exposed so I decreased) and contrast (I increased it a little, more drama)

Step 4: Play around with effects. Here we have Boost:

and Vibrance.

I went with Vibrance. They also have some seriously far out ones, and traditional like Sepia & B&W. For a lot of fun, go with Lomoish.
Step 5: Soften if needed. Sometimes I like doing this, especially if the colors are strong like in this one. This is the Matte effect.

Step 6: Rounded corners and add text. I love the text options, and I keep wanting to find reasons to use the really wild ones. Still, the ones I like best look like neat writing or retro typewriters. I was feeling purple, so purple text it is.

Step 7: Shrink to Fit. My Blogger header has a 890 pixel width. To make sure that it scales appropriately (instead of letting Blogger do it for me) I save the file as 890 pixels wide and let it scale the height.