Here you can set your anti-aliasing and make the game look as good as possible if you have a NVidia graphics card. I've asked the creator of the tweaker about this and he said there's nothing he can do on his end about this so you'll just have to settle with the non-Steam game method.

Screenshots do not work by launching the game normally through Steam for some reason.

You can use screenshots by adding a non-Steam game shortcut of the game's T3. For this section of the guide we will just be focusing on editing the 'Sneaky Upgrade' portion of the Sneaky Tweaker. In this section I will go in-depth into the Sneaky Tweaker and how to set your game up perfectly. An equally as important step in the overall guide to getting the game to run perfectly on your system. In the next section I will explain the tweaker itself. You can enable and disable any of these mods any time you like. All of this stuff is optional, so unless your HDD is super small or your internet sucks really bad, there's really no reason not to get Fat. It contains a few completely optional mods and the incredible John P texture pack. One contains the editor which you will most likely not care about, one of the upgrade patch containing only the patch itself and its tweaker but the Fat Edition is the one I recommend most. Important Note: You will find three versions of the patch. The patch is upgraded reguarely so be sure to check the main page for the latest version. A one-patch beats all fix to have Deadly Shadows run flawlessly on your system. Similar to Thief Gold's TFix and Thief 2's Tafferpatcher, Sneaky Upgrade is the equivalent for Deadly Shadows. The game may run well enough on its own, but then you'll get lots of gltiches, no HD support, tiny FOV among many other issues. Sneaky Upgrade - The Modern Compatibility Patch

Thief deadly shadows mods