This is the sort of issue that software architects run up against all the time. As per usual, there is no ideal solution. You need to select which compromise is most acceptable to your business.
To summarise your problem, most of your image processing software is written in .NET. You’d like to run it client-side on mobile devices, but there is limited .NET penetration on mobiles. The alternatives with higher penetration (eg. Flash) would require you to re-write your code, which you can’t afford to do. In addition, these alternatives are not supported on the iPhone/iPad.
What you ideally want is a way to run all your .NET code on most existing platforms, including iPhone/iPad. I can say with some confidence that no such solution currently exists – there is no “silver bullet” answer that you have overlooked.
So what will you need to compromise on? It seems to me that even if you redevelop in flash, you are still going to miss out on a major market (iPhone). And redeveloping software is extremely costly anyway.
Here is the best solution to your problem – you need to compromise on your “client side execution” constraint. If you execute server side, you get to keep your existing code, and also get to deploy to just about every mobile client, including the iPhone.
You said your server power is limited, but server processing power is cheap when compared to software development costs. Indeed, it is not all that expensive to outsource your server component and just pay for what you use. It’s most likely that your application will only have low penetration to start off with. As the business grows, you will be able to afford to upgrade your server capacity.
I believe this is the best solution to your problem.