It will be best to pick a specific pump model for each month, like Wayne 866 or Martin & Schwarz 80 and even the lesser known models if you can find a person who owns one and will supply photos. Then ask others to add their photos.
X2
Braden, I agree with Jim. Like I mentioned in our PM - Maybe just do a different model number each month. I wouldn't be to broad, or you will run out of new pump of the month threads in a few years. I think there is enough people out there that have a lot of types of pumps... even several of the rare ones (if they are willing to share). There are a few super-rare pumps that might be trouble, but people could just post old pictures then.
But I also had another idea. If you do a POTM for an extremely common pump (e.g. a Tokheim 300, etc.), maybe you could ask for restored one month and un-restored a different month.
But this is your baby - do as you wish. Just a thought. Thanks for doing it.