There are two possible reasons for this:
1) Someone released a map with invalid texture size or somehow a texture has become corrupted. In Swat 3 textures have to be 8, 16, 32, 64, 128, or 256 pixels (in width or height) whereas other games aren't so restrictive. The only way to fix this is to keep track of which maps cause this error and delete them from the MISSIONS folder
2) You are trying to play a newer map that was designed for use with the Last Resort mod without having it . The Last Resort mod allows larger textures with a range of 8 to 1024 pixels. Fix: download and install the Last Resort mod. You can toggle this mod completely off with the custom launcher so you can go back to vanilla Swat 3 any time you want.