Recommended workflow to retain maximum quality of an image for printing

[Ofnuts]

See here for a background understanding of the size of images.

If you say your image is 42"x56" @72DPI, it means it is 3024*4032 pixels, and printed at 23.5"x29.5" image this will be around 130DPI (possibly around 100DPI depending on how wide the frame is in the original photo. Not very good (that would be 300DPI or better) but not bad either for a painting (which people are not going to pixel-peep).

Typical workflow is is to make the image exactly rectangular (the slightest angle in the camera makes it trapezoidal):

• Starting with the initial photo (frame included)
  
• If the colors don't look exactly right, and if there is something in the picture that is naturally gray/white, insert a color correction step here.
  
• With the crop tool, crop the image to the smallest rectangle that still contains the whole painting (this can leave some bits of the frame).
  
• Start the perspective tool, and set it to Direction: Corrective and the Clipping: Clip
  
• Drag the four handles in the corners to the 4 corners of the painting, and apply the transform. This will put just the painting in the canvas
  
• If there are optical distortions in the picture, there can be small bits of the frame left on the sides, it yours to decide to crop the picture a bit more at that point or to keep these or to fix the problem by other means (cloning....)
  
• Compute the aspect ratio (width/height) of the picture you obtained with the AR of the original picture (0.8) and scale the image vertically or horizontally to restore the aspect ratio.
  

Otherwise, all images taken with a smartphone are in sharp focus, because the smartphone lenses have a very small focal length and this gives them a very large depth of field (advanced smartphones use post-processing to obtain the same background blur (aka "bokeh") that you have with full-size cameras). But they are also over-processed, to look right on the phone screen, and not in print (color balance, etc...). You "sharp focus" could be my "sharpening artifacts", not too visible on a smartphone but an eyesore once you see them.