Unfortunately just a spectrum shot like that will not be enough for a positive ID. There are several things that can make that shape on a spectrum display. The addition of a waterfall would help greatly as it shows the trend over time.
Also, time and date (in UTC) would help.
Yes, what you have there is quite possibly WEFAX, probably NMC out of Pt. Reyes California. These kinds of stations send their images on a specific schedule, and the time could be used to confirm that NMC was indeed active at the time you saw/heard this signal.
However, this frequency has also had STANAG 4285 traffic before, and your spectrum image could also be that. A waterfall of the DDC window would clear it up between the two possibilities I have mentioned.
As for software to decode the images, there are several out there. WinRadio themselves make a "Digital Suite" that will interface seamlessly with your Excalibur. It works, is very stable, and has a fairly easy to use GUI. However, for the price it is pretty limited as to what modes it does. A free software is MultiPSK, it is very capable decoding many modes and working well, but the interface is not the most intuitive in the World. A Google search for "HF FAX Software" will show a lot more options, there are probably hundreds of possible programs out there, some pretty good, others pretty junky.
T!