What I would do is take several pictures at 640 x 400 in software mode. (You can use windowed mode if you have to). Once you have done that, scale the image(s) you like the most using an image editing program like GIMP, paint.net, or photoshop.
There are a couple of ways to scale the image, so I would look at multiple scaled down images before I saved the one I liked the most. If you use GIMP, I would first convert the picture to RGB so it looks nicer when you scale it down, you might have to do that in with other programs as well. When you scale it down, you have to scale it down to 160 x 100. In GIMP there is an option for interpolation, which is the option to play around with until the image looks right. Try cubic, linear and sinc methods to see which one looks the best.
After you scale it down you have to convert it back to the palette. You can do this with your image program (if you have the srb2 palette already imported) or use XWE or SLADE to convert it. (look at
this page for more information on the palette) From my experience, XWE does a really good job at converting it, the colors it picks are usually very close to the original feel of the picture. SLADE is probably pretty good, I haven't personally used it for that since SLADE came around after I stopped making wads. GIMP is pretty good for converting, it works well enough for converting scaled down pictures. Using the WAD editor probably going to be the easiest method.
There you go! Try to look for interesting geometric areas to take pictures of.