In project settings (next-gen project), under 'Project settings', I have no luck setting a custom project avatar. I can select one of the existing ones, but if I attempt to upload a custom image, or drag in an image, I get an error when pressing "Select" to set the new avatar:

Is this feature broken, or are there some limitations to the image that I'm not aware off (I've tried jpg and png, and different sizes from 100x100 to 1500x1500)?