If you have designed your logo and want to use it for your Blogger blog, you can easily get it done by using the Blogger's Page Header gadget.
The "Page Header" gadget is the gadget that is responsible with the display of your blog's title and description. The gadget also makes it possible for you to use an image instead of your blog's title and description. Hence, it is the gadget needed for you to add a logo to your blog.
How To Get Started
Log in to your Blogger dashboard and click on the blog you want to work on.
Go to Layout or Design
Right on the page elements page, you will see the "Header" as seen in the picture below
Click on the "edit" next to it.
This will bring up a new window similar to what you are seeing in the picture below:
Click on the "choose file" button to upload the logo you want to use
Once chosen, select the "instead of title and description" as seen in the picture below
Do Not tick "shrink to fit". (You can tick it if you are uploading a wide banner as your blog header image)
View your blog and you should see your logo been displayed in your blog header.
NOTE: If you are uploading a wide banner, you can tick the "shrink to fit" so that Blogger can resize the image for you to fit perfectly into the blog header. Nevertheless, it makes more sense if you can make your wide banner not to be more than the size indicated below the "shrink to fit".
I hope this helps...
You can use the Pixlr editor to create your logo or your header banner online. I hope to share a video tutorial on how to do this sometime soon. Just subscribe to this blog for you not to miss it.