Hi All, I'm a bit late to the party. I've had this same issue this week. I downloaded one of the default images to see that it was indexed colour and 8 bit. I'm not sure if this is also the issue as I stopped testing after I converted all my images and found what I believe to be the issue.
When you select an image it will be added to the project files. However, the code that is created from the UIBlocks shows it is trying to load an image from res/img/.... As soon as I uploaded the image to this folder, via the web terminal file menu, my issue was resolved.
I realise this is a bit late for the others, but, hopefully, new comers may find it useful.