either light room or photoshop is changing the original size of the piece of artwork - for example - one piece is 24 by 18 - but when I go to image size it is 24 x 16- I have not cropped it-
Also when I go to image size, I do not see Scale styles, constrain proportions. I am using Photoshop CC.
Your question seems a little unclear. You say the photograph of the artwork is 24 x 16. That is a 2:3 aspect ratio, which is characteristic of most digital SLR cameras. If you want the copy to be 24 x 18 then it will be necessary for you to crop the image. If you are using Lightroom you can control the exported size of your image in the export dialogue. If you are using Photoshop CC as you indicated, you will have to use the crop tool and then adjust the size appropriately using the Photoshop tools.