I agree with @TheOnlyAaron. Show us what you are trying to do. And you’re trying to do this in SketchUp Free (web) as your profile indicates you are using?
Since you are building a 3D model in 3D space in front of a 2d image, that pillar will never go behind the pillar. If you need that then you are best off modeling that pillar in 3D space in front of the mirror.
Can you share the entire photo? It might be possible to use it with Match Photo. If it will work you could use it to guide you in creating a model of the existing structure and then use the image as a texture. After you have that modeled you can add in your other elements.
Please correct your profile to reflect that. The tools you have available in SketchUp Pro are different than what is available in the free web version.
As a 3D modeling software, the idea of “Masking” does not exist. SketchUp shows the things you model in 3D space. If you want something in front of another thing, you need to create those items and arrange them in 3D space.
Create a second image in an image editor with the background deleted and move it in front of the model. It’s a simple enough thing to model, though. Easier than dealing with multiple images.
Or, create two scenes, one with both the image and the model, aond one with only the image. Send to LayOut. Put the two views on top of each other (only the image topmost) and crop the upper view at the edge of the column, if necessary with a clipping mask. This was done in that way: