I am trying to crop a large group of png files that have been imported into LR. The dialog for exporting does not allow for resizing when exporting to "Original" file format, and the crops that I set in LR are lost when the file is exported. All works fine if I export to jpg or psd.....
Lightroom cannot export png files (other than the unedited unchanged original); if you want your edits (which includes the crop) and resizing to be in the exported file, JPG, PSD or TIF are your options.
dj-paige has given the correct answer.
Lightroom could not even import or view PNG files until v5 (if I am correct?)
You will need to do all editing, cropping and saving of PNG files in Photoshop or another capable editor. (You can Ctrl+E: Edit-in.).
Lightroom, not being a 'pixel' editor, cannot edit files with 'transparent' areas.
You can export from Lightroom as JPG, but any transparent areas are converted to white pixels and the file will no longer show transparency.