is it possible to block the image addition on the design to be limited to the image masks plain shapes ?
Am not sure what you mean.
Can you explain in more details. Image masks requires images to be put into them; if you want only shapes, then you upload only shapes into the admin, and blocking photo upload, the customer will not be able to upload photos. Am not sure I get what you mean.
I would the user to be able to add an image only in the shapes, not anywhere else.
Also is it possible to add a "placeholder button" in those shapes to show the user he has to click to add an image ?
Oh I see. We do not have that option but the closest you can do is disable image clicking. So they can only drag and drop unto your shape.
As regards the second question, yes you can make the mask clickable. When you make it clickable, customer clicks on it and they can select their photo, upload and it gets loaded straight into that shape. But you might have to place a reference text label saying click here to place photo.
Once you have a plain shape on the canvas, click it and on the admin tools, you will see a toggle button to set it as a clickable mask.
Ok, but your solution blocks the "Clickable mask" option... and the user can still drop on the design.
Would it be an easy feature to add ? I'm web developper, maybe I could somehow help you.