Although it's likely a moot point by now, in this instance anyway, the photo issue can be dealt with at the visa desk on arrival at Pochentong Int'l Airport. The cost varies depending on the person dealing but was around $1 or $2 each the last time I did it. It ends up just being a photocopy of your passport picture but seemingly good enough for them.
Someone may have more up-to-date info on the visa on arrival at Phnom Pehn's airport, but I always found the best way, despite costing a few extra dollars, was to get an e-visa ahead of time. By doing the e-visa it can save a lot of potential palaver with very long and disorganised queues at the visa desks, plus much less opportunity to be ripped-off.
With an e-visa you just bypass the melee and head straight to the actual immigration desks. Just make sure it's one with an 'e-visa' sticker on display otherwise you may queue in that line for nothing.
e-visa
If you get the opportunity it's well worth scanning some passport photos and then emailing them to yourself so they can be accessed anywhere (e.g. for the e-visa application), as well as a scan of your passport main page and any other important documents, such as insurance. The more modern method might be to save them to a cloud, plus your phone too.