The appears to be a bug in PictureMarkerSymbol.angle for version 2.5 of the Flex API.
In version 2.4, when applying a rotation to a picture, it is rotated around its center. In version 2.5 it appears to be rotating round round the top corner. Adjusting the offsets does not fix the problem, as it does not appear to be rotating round the offset point.
The attached images demonstrate the problem. Each point is a compsite marker with a simple diamond and the rorate picture marker. In flex2-4.png, the two markers are aligned, but in flex2-5.png the rotating picture has drifted off by an uncontrollable offset.
Until this is fixed, it looks like I'll have to stick with version 2.4.