Photoshop :: Resizing Photo Of Paintings So They All Are Same Height?

Last week I have made photo's of 140 different paintings. Some paintings were very big, some were very small, some where rectangle shape, some were square shape. All paintings were set upon a stand and I photographed all of them from my tripod. So you can understand that on some pictures I have large paintings and on some photos I have small paintings.

Now I want to remove everything around the painting so that my photo ONLY shows the painting (even the border around the painting has to be removed).
 
First I select only the painting. Then I inverse my selection and remove it. So I'm now left with a photo of only the painting. So far no problem.Then I select the painting and with CTRL T I place the 'reference point location to the upper left corner and than set the height to 2000px. The painting now becomes 2000px high, but the width isn't corrected so I click on the 'maintain aspect ratio' and the painting is okay. This is now done with (for example a square painting).
 
But when I now perform this action on a rectangle painting, the end-result is a square painting.