You need to download and install the Diablo 2 High Resolution Mod
The downloaded "patch" consists of 4 files:
Copy D2MultiResGame.exe, D2MultiRes.dll and D2MultiRes.mpq to your Diablo II directory. Run D2VidTst.exe (found in your Diablo II directory) and choose the DirectDraw option.
To run, start D2MultiResGame.exe. Diablo II will start in the standard resolution, when playing a game you will be able to switch to a new, higher resolution. To do so, go to the Video Options menu and select the Resolution menu item. This will bring up a new panel that will list your available resolutions. Click the one you wish to play in and Diablo II will switch to that resolution.