I need a simple way to capture images from smartphones without writing an app for every Operating System ( android, ios, winphone ).
Is there a way to capture images from a phone camera ( obviously without using OS API ) on a web page ?
Short answer: no.
Your best bet is to use Apache Cordova. However, note that the web page needs to be wrapped in a native WebView as an app, as it is not possible to take a picture using the phone's camera directly from a web page. The advantage of using Cordova/Phonegap is that you do not need to write custom code for each platform.