Hello,
You can use PixelMath. In RGB/K field use the max function, something like this:
max(transparent, target)
The names transparent and target are examples of the IDs of the images, you need to replace this with your IDs (use the Expression Editor for easy editing). Select "create new image" an apply (F5 o blue square button). Note that the two images must have compatible geometries (be the same size).
Best regards,
Enzo.