Sometimes, if an image is above another, but you lower the opacity it goes below another image.
When this happens, it's not that changing the opacity value by the slider doesn't work, but it also just seems to make it change order in this layer.
Possible starting point for solution: To have each of the images at a different layer, and switch layers on click.
Masks are already fixed at a high layer though and I think it also happens to them.
Sometimes, if an image is above another, but you lower the opacity it goes below another image.
When this happens, it's not that changing the opacity value by the slider doesn't work, but it also just seems to make it change order in this layer.
Possible starting point for solution: To have each of the images at a different layer, and switch layers on click. Masks are already fixed at a high layer though and I think it also happens to them.