Re-texturing 3D model in Wings

[MonteMNE]

Hello,
I am new in using Wings 3D so I beg you for help. Here's my problem: I import a 3d model that already has been textured in some other 3d software. I delete all materials in "Outliner" window. Then I want to UV map it on my own. I select faces I want to do, and click UV mapping, but instead of normal window that appears with model I made entirely in Wings, that default texture appears immediately without anything, (sometimes with them in shape of triangle taking up exactly half of texture) and that default texture doesn't show up on faces I selected. What should I do? I just want to UV map model in Wings 3D that already has been textured in some other 3d modeling software.