I mean at the place you can see in the screenshot, there is no way to progress.
My phone can't open tga files, but I think I know what you're talking about, because I ran into this problem, as well.
IIRC, you should go back to the beginning of the level and check the corpse of the sewer worker there. There should be a note which gives you a clue.
In case you're frustrated and don't want to search the whole place, here's the solution:
There are some explosives in the pipe above the main tunnel right at the start of the level (before you open the first gate). You'll need to crouch into one of the pipes and find a ladder to that pipe above.
I haven't played the mod in quite a while, so I'm not sure if they changed it in one of the later versions.