gyROS:
Your Paint is going to be a nice test box to find drawing problems. We have some issues with Drawing(although our Devs lately have fixed a lot of bugs about Graphics),i.e: pen width (
http://www.reactos.org/bugzilla/show_bug.cgi?id=2761),among others(as problems with our Mouse Messages).
You must develope your Paint and check if it runs nice in Windows:If mouse click works in Windows,if palette color works in Windows, if your features works in Windows, then it should work in Reactos.If doesnt, dont try to change the code finding another way to make it workable in Windows and in ReactOS.Or worst,finding a way(a Hack) that would make it works in ReactOS but not in Windows.
To sum up: Finding an alternative way because a Bug will hide the Bug.And we need to find it(related to:)
gyROS wrote:I'm planning to kick out the StretchBlt anyway, replacing the currently used bitmaps with IPicture instances (they offer save to stream functionality, so why not use it)
If one feature works nice in Windows,but isnt working in ReactOS then it´s a Bug

...report to our Bugzilla.Anyway, if you have doubts about if it is/isnt a bug ask any tester(i.e: me) to confirm it.We would be pleased to help you.You can find us in IRC:You can use Chatzilla-Firefox Add-on to join us.
As you are the Paint developer you can easily point our Devs which function(of your code) isnt working as it should(as it works in Windows), that will help them to find the Bug in our ReactOS code. Indeed,if you are enough skilled(and if you are working with WinAPI you are),you can find that guilty function in our Doxygen and find how we are (wrongly) implementing it and why it isnt working properly.
Also, maybe you can hang your code in SourceForge so we can have a look of your code.Is it GPL?.Or at least the .exe to confirm bugs

Thanks for your work