A custom logo would reside in the mxd of the print template. There is no coding needed.
Create a template with the logo on it then add the mxd to the location where your print templates are for your arcgis server. You need to make sure the WAB is pointing to the right print service. Once that is done the template should show up in the drop down box in the widget for you to choose to print with that template.
Here is a tutorial: ArcGIS Help 10.1
Thanks for the response.
I'm purely in an AGO environment when I use WAB. I am not using an internal ArcGIS Server, so I don't have customized print services. Is there a way to setup print templates in AGO? Thanks!
No, I don't think so. The MXDs have to reside in a folder on the server I order for the print task to be able to access them, and uploading MXDs is not supported in AGO.
No, unfortunately it hasn’t been implemented, and I’ve never found a work around.
They list the status for my support request regarding this issue as “not in current product plan” and the severity as “low”, so it doesn’t appear it will happen anytime soon.
You can, however, vote for the idea here:
Once ArcGIS Server has been fixed to allow printing support for MapImageLayer then there will be a way to do this but not until then.