There are four great options you can go for:
- HTML5 + CSS 3 + AJAX
- Silverlight
- Flash
- Java
HTML 5 + CSS 3 + AJAX (for the surrounding site.)
Since there are many graphic elements, and these upcoming standards are quite new, it would be difficult to get everything that you would like to achieve in this language, you could create most of the site in this, but the image editor itself is not easy enough to develop in comparison with other languages.
Silverlight (If you have a lot of time ... Bad choice, for now ...)
For many users, this is not installed by default, although support for installing it is reasonable (Moonlight on other operating systems, Silverlight may work in different browsers), if you want to reach the general public, you can skip this for now. This will allow you to learn C #, .NET, WPF and other Microsoft technologies so that you can move, which will require a lot of training time from your company.
Seesmic, the second popular Twitter client, however, seems to use Silverlight to quickly install and update its desktop application in the browser, so it may not be too difficult to require Silverlight from your users. Another bonus to learning C #, .NET and WPF is that you can use the technologies you have learned to write desktop applications. (Which again may work on other OSs, Mono support is getting good)
Java VS Flash (Stay if you have no reason to change!)
As you can see on Google Trends: Java, Flash, Silverlight , you will notice that Java and Flash are on the same level, so they are both supported by your users. You can best stay with Java because you have experience working in that language, and not forget that you can reuse the code you wrote. Another bonus when staying with Java is that you do not have to switch between software to develop different parts of your infrastructure.
You can search Java VS Flash in Google, and only when you have a reason to change, you have to do it, it makes no sense to study something with the same (or maybe worse) capabilities when the thing you use in great for doing this job right now. There is nothing wrong with learning something new when you have time for it, but you will have the disadvantage of being new to it ...