See


Report #:  43020   	Status: Reported
TBitmap.Canvas.Lock is a MUST in multithreading applications
http://qc.embarcadero.com/wc/qcmain.aspx?d=43020


Jan Derk:
Contrary to what the Delphi documentation says: one must lock all 
canvasses, both source and targets, in threads even if you do not share 
them with other threads, because the GDI object caching mechanism that 
runs in the main thread may just release your DC when your thread is 
not expecting it.


TJPEGImage.Draw() is not thread safe
http://qc.embarcadero.com/wc/qcmain.aspx?d=55871


-- 
Alex