As I can read it from this article the maximum square bitmap Flash can handle is 4,095 x 4,095 (which results in 16,769,025 pixels and bla-bla-bla). So, they say if I try to go
look at this http://kb2.adobe.com/cps/496/cpsid_49662.html
the actual size limit is 16,777,215 pixels (the decimal equivalent of 0xFFFFFF)
I think these limits refer to what you can create in flash via the bitmap class, not what you can load externally...but I could be wrong.