I created a custom background image, 800x600, saved as .png. Then I navigated to images in ACC, clicked "new", then tried to import. If someone has a screenshot to share showing how you upload a new image file for this custom background, I'd appreciate it. I'm doing something wrong because the uploaded file is not working.
Go to Administration > Configuration > images and then upload your image with a name and select the file from the icon highlighted on the right side icon.
Then save the images. Logout from the console.
Go to your workflow properties and in the advanced tab select the background imag
Once selected. Logout from your console and go back to your workflow you will see the updated background image.
Let me know if that helps.