Here is a way to insert an image into the header of your blog.
This code is for the Binary theme, but you should be able to get an idea and then just adapt to the theme you are using.
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It is of course important to get all images the right size, but for those in the body of a web page a simple resize before using will usuallly be sufficient. There are also ways of specifying an image size using html or css code, but of ...
How To's for using the new postach.io blog with Evernote - Simple How To guides on setting up a Postach.io blog - This is not an official Postach.io blog, although many of the guides have been collected from different scattered postach.io sources