The problem only seems to happen when "Lighting" is enabled within the game's graphics options (anything other than "Off"), and only when OpenGL is used for the graphics renderer (the DirectX renderer seems to take PNG format screenshots just fine).
Anyhow, this is what I see when taking a PNG format screenshot, using the OpenGL graphics renderer, and when "Lighting" is enabled in the game (windowed or fullscreen mode, same results):
http://jasonwilliams400com.startlogic.com/snor/weeds/Temp/OpenGL_with_Lighting_Enabled.png
Is it just me/my video card, or is this a small buglet in the game?
(Edit) P.S. I like to use the OpenGL renderer (as the game seems a wee bit smoother), and prefer PNG format screenshots (due to the relatively poor quality of the JPG format screenshots, even when "screen_quality" is set to "100" within the game's advanced graphics options).