I can agree that it has pretty much zero workflow and is generally highly unintuitive.
As for putting your graphic in your skin... have you found the skins page yet and figured out how to upload one yourself? I suggest grabbing some free skins and learn about uploading and applying them. After that you can start examining the skins prior to upload and modify one with a graphic.
Note that most skins also make use of the Logo option whereby you can upload a graphic in the Admin settings to appear where the DotNetNuke logo normally sits. Using that is slightly different from using a skin that already contains a hard-coded header graphic.
Important things you need to learn about are:
- host vs admin skin uploading
- ascx vs html skins
- tokens
Some info: http://www.dnncreative.com/HowtouseSkinsandContainersinDotNetNuke/tabid/212/Default.aspx
Check out these videos. I haven't watched them myself, but they're free and would seem to cover the main points: http://www.dnncreative.com/Tutorials/DNNTutorialsforBeginners/DotNetNukeQuickStartGuideVideos/tabid/73/Default.aspx
More free content there: http://www.dnncreative.com/Home/FreeContent/tabid/101/Default.aspx
There: http://www.skinningtoolkit.com/INTRODUCTION/tabid/84/Default.aspx
And keep checking out sites that people in the forum refer to.. browse through back-posts in the DNN general forums. Be prepared to register at a couple of hundred websites, be ready to spend money on modules, be ready to learn a whole pile of skills you never thought you'd need to know... and always wear a helmet!
It is worth continuing to check out other portal systems as well, because each one has strengths and weaknesses and you have to make the right choice early on.
Rob