It seems a few members have had problems attaching photos using their cell phones on this site, first of all I am sorry your having problems but if this happens see if your browser is listed on this site
http://tracker.phpbb.com/browse/PHPBB3-11148 below is a list of go no go browsers they had listed on that site.
[OK] HTC Desire Android 2.3.5
[OK] HTC Sensation Android 2.3.4
[FAIL] HTC Sensation Android 4.0.3
[FAIL] HTC One S Android 4
[OK] HTC One S Android 4 (Firefox)
[FAIL] HTC HD2 Android 4.1.2
[FAIL] HTC EVO 4G LTE Android 4.0.4
[FAIL] Nexus S Android 4
[OK] Samsung Galaxy S2 Android 4.0.4
[OK] Samsung Galaxy S2 Android 4.0.4 (Opera Mobile)
[OK] Samsung Galaxy S3 Android 4.0.4
[OK] Samsung Galaxy S3 Android 4.0.4 (Firefox)
[OK] iPhone 4 iOS 6.0
[OK] iPad 2 iOS 6.0 (10A403)
[OK] Samsung Focus (SGH-i917) Windows Phone 7.5
My advise would be if your phone wont let you post pics I would try installing a different web browser and try again.
Hopefully the browsers will correct this problem or PHPBB will come up with a official fix.