Extract portion of WMS raster layer

Most of the GIS desktop apps can do this kind of operation. In the "old" geoprocessing library "Sextante"* you can clip a raster with a vector layer with the algorithm called Crop Grid With Polygon Layer.

In QGIS processing framework you can use the GDAL provided algorithm "Clip raster by mask layer"

*As 05/2018 lot has changed since this post was written. In their first version sextante were working with gvSIG and begins to work with QGIS. Now sextante itself lost its name both in QGIS and gvSIG. It's fully integrated into QGIS and renamed as QGIS processing framework. In gvSIG is known as gvSIG Toolbox.

You can't clip, edit or save streamed layers from WMS-Servers. What you need is a WFS-Connection to your Orthophotos, where you can temporarily cache data.

The other alternative would be (if you have only one area of interest) that you make a screenshot from QGIS and save your WMS-Ortho Map as png. After that you just have to digitalize your screenshot (search for tutorials on this site and the net).

This should work with PYQGIS

wmsLayer_name = 'landsat'
url_with_params = 'url=http://irs.gis-lab.info/layers=landsat&styles=&format=image/jpeg&crs=EPSG:4326'
wmsLayer = QgsRasterLayer(url_with_params,wmsLayer_name,'wms')
clipBbox = QgsRectangle(-102.33797581141405431,21.7442917848168058,-101.99476739437655226,22.00576212354055627)
wmsLayer.setExtent(clipBbox)
QgsProject.instance().addMapLayer(wmsLayer)

And then you could save local (.tiff,.pgk,etc) with QgsRasterFileWriter