Strain oil on canvas on cardboard
 

Good day!
I would like to know whether it is possible to correct the deformity,the picture is curved,it can be fixed?

DREAMer, is quite fixable. Find a good restorer and give him the picture on the fix.

Correctable. Even in the case of hardboard, and even cardboard...

Know only one way proven and does not require expenses for services restorer..Ordered the subframe size of the picture plus 5 mm on a side , carefully glue the cardboard to stand under the weight for 10-12 hours until completely hardened glue.As the glue is good to use so-called liquid nails ,as cargo book.The subframe is desirable to do with a transverse crosspiece for rigidity if the size of the cardboard more 30x40.Can't be glued, if the cardboard is not very rigid and deformation is negligible .It helps to hold the subframe to the frame and lock it.By the way ,so it is recommended to mount in a frame painting on cardboard , even if the cardboard has no deformation.When impasto painting (like you) need to do everything slowly, to avoid cracking and lag paint layer.As far as I know all other methods of alignment of deformed cartons will only lead to damage to the paint layer.