havent seen the picture yet but i guess u have set r_displayrefresh 100 and the monitor cant handle 100hz on that resolution..

the resolution is not supported.

make a custom resolution with the same size and try different sub-timings, or pick a different resolution what's supported by your display.

you didn't say the exact model number, but you may try this:

/seta r_customwidth "1440"
/seta r_customheight "900"
/seta r_displayrefresh "75" (or "60" if 75 fails)
/seta r_aspectratio "2"
/seta r_mode "-1"
/vid_restart

you can put it into a cfg and execute it from the console (/exec whatever.cfg) . Please note that you don't need the "/" signs at beginning of each line if you go with the cfg approach

If something goes wrong (black screen) you can go to windowed mode with alt+enter

edit: but LCD screens perform the best at the native resolution. If you need to go lower because of performance issues and there is no supported lower resolution available , it's advisable to go with "GPU scaling" at the native resolution of your monitor to avoid increased input lag.