Hi Deb,
Go to any CVS farmacy store. They have a machine that has a setting to print you 3.5 cms X 3.5 cms photographs. You just have to tell them in inches because they do not understand the centimeters. They can give you 1.4" X 1.4" photographs. I found none of the other stores like Walgreens, Costco, Walmart etc. have that. Most stores and photo shops give you standard passport size 2" X 2" photographs. CVS farmacy is the best place to go for photographs of that size. Just tell them you need 1.4" X 1.4" and they will do it. If the operator doesn't get it then tell them that their machine has a setting to produce that size. Some people might not be aware that the machine has that size. They all understand passport size though.
Regarding number of photographs, you need 5 photographs for each applicant. While 2 of these photographs are to be affixed to form A, 3 have to be enclosed as attachment alongwith your application. Since you submit your application forms in two sets, you are required to affix one photograph each on form A for each set. The CVS store will give you 8 photographs for $7 or something. That is sufficient for your need.
According to the CGI Houston website, "Two photographs should be affixed in the boxes on the application form and three photographs should be kept in an envelope and attached to the application."
Check the following link for instructions on photographs:
http://www.cgihouston.org/OCIProcedure.html
I have read some people saying that they submitted the 2" X 2 " standard passport size photographs without any issues. They would have done it because it is so hard to get 3.5 cms X 3.5 cms size photo in US. The stamp size is too small and will not work for you, while 2" X 2" is bigger size.
I was just playing it safe, so I got me a size closest to the photo requirement. 1.4 inches is equal to 3.556 cms which I figured is same as 3.5 cms. I believe 1 inch = 2.54 cms. It will be a waste of your time to try getting that size from photo shops because I already went through the pain and found that only CVS store could deliver that size.
Good luck to you.