1. Check how wide your image should be (330px for RMRS web-site)
2. Optimize image to a lightest size possible preserving good quality
3. Create a new post or page
4. Upload the image to the post, hyperlink it if needed.
5. Switch to text mode of the editor and copy the whole code. Don’t save the post/page – just abandon it.
6. Go to AppearanceWidgets and drag and drop new ‘Text’ widget to a desired position in Primary sidebar
7. Paste the code and save. You may want to provide additional CSS class to it or/and Google Analytics even tracking code.