Online Photo Editor! - a good photo editor to collage your photos online.
If online means to display a collage in a web page, then one way I've done it is to convert the collage into a jpg of the whole collage and put that collage picture in jpg form into the web page. This may mean photo reducing the size of the collage down to near the size it will be on the web page in order to get the best resolution, however. Compromises are a part of life.