Currently, the image format that a WMS defaults to is JPG. JPG is best used to display photos and the like which contains millions of colors. It compresses well, however it is lossy and doesn't allow transparency. It is a good format for a aerial photo basemaps.
For WMS layers, which are usually placed over a photo basemap like GoogleMaps, GIF is a better format.
GIF is lossless and works best for images with a fixed number of colors (as is usually returned by WMS layers). GIF allows transparency.
And for the record, PNG has similar properties to GIF, but older versions of IE have problems rendering PNG layers, so is not a suggested format.
So I suggest that the default format offered by the Openlayers WMS is GIF format instead of JPG as is currently the case.
OpenLayers
