I think you could do it and they would accept but you just have to expect that they might get folded over and a bit damaged because they’re flimsy. A few years ago I wanted to do the same thing so I got them printed and just bought a piece of posterboard, nothing fancy, and spray glued the photos onto that to make it a bit sturdier. It was easy and they got delivered fine.

The USPS says post cards and letters must be 0.007” thick.

How that can be measured I shall leave as an exercise for the reader.

Link (click on “Rules & Restrictions” to see size limits)

Here are the specifications for USPS Postcards

Thanks @blueiiznh . So a 4×6 home print glued on thin poster board would not exceed the maximum thickness.

@prioritymail thanks for the idea!

Photos are susceptible to damage by water, so be aware that your pics might be damaged if hit by rain, while in transit

Some photo quality printers are capable of using photo card stock, if you have a printer that is so equiped you can print them straight to the stock.

Another option is Walmart or Sears photography section, they have the capability of doing your prints on postcard stock for you. That is if you want to avoid having to paste the pictures to card stock.

Thanks everyone!

@zenvelo You’re welcome!

I tested this out and it worked perfectly. There was some small damage to the photo, and I only sent it within the city. I simply had 4×6 photos printed, addressed them with Sharpie and put a stamp on it, no extra backing for thickness. I measured the thickness of the photo print (from Walgreens) using a digital caliper and it exceeded the minimum USPS limit. (USPS says on their website that index card thickness is acceptable.)

So all in all, I would do this again if I need to make a cheap and quick and local mass mailing as was the case here. If it were being sent a greater distance, I don’t know how it would hold up. I have never had a problem with mail getting wet, but the photo paper is not that durable and I think friction with other mail can lead to damage. There are a lot of websites that print “real” postcards inexpensively if you have the time to wait for it to be shipped.