I clearly sounds like a bug. I tried this on the latest code base and it seems to work as expected. I'm not sure when this bug was fixed though but you might want want to try to get the latest patch and upgrade the ui add-on if you are not already running the latest versions.
I have a pagetype that contains some CultureSpecific property's.
The same pagetype also contains the following property:
Note that the property is Required and non-CultureSpecific.
Now i do the following steps:
When in forms view, i can see the property is grayed out. It also tells me the property is 'shared between all languages and can only be edited in the master language'.
However the image component does not seem to have a value, because i only see a gray readonly textbox.
So i translate the CultureSpecific fields and press Publish, i get the following message:
The Image (Minimum of 160 x 160) field is required.
This offcourse should never happen, since it's clearly indicated that the property can only be edited in the master language.