Can't apply alpha maps to landscape's brush settings?

After several years on Maya/Arnold, I wanted to test Unreal for a personal project. The learning curve is great so far, but I’m stuck for quite a long time on a dumb issue: every time that I want to apply a custom alpha in my landscape Brush Settings’ texture attribute, Unreal automatically chooses the checker texture.
My custom textures are all .tif, but I tried to convert them as .png and .psd thinking it would work, without any positive result unfortunately. I saw countless of tutorials where people were able to drag and drop their own custom textures, but I can’t manage to do that.

Would you have an idea of the issue? Are my textures not compliant to Unreal for a reason?
Thank you very much!