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Batch conversion from JPG to PNG
Ofnuts, it is a BIMP issue. BIMP adds an alpha channel when the 'Change format and compression' and PNG type procedure is used. That accounts for the increased file size. There is an example in an earlier post.  There are no options in the BIMP dialogue for bit depth or format.


I spent hours trying to find a work-around, no good adding a 'flatten' Gimp procedure, the BIMP export is the last procedure in the pipeline and adds that Alpha channel. To complicate matters BIMP has a list of reserved terms that can not be used. That includes 'save' which rules out pdb.file-png-save.  I can get round this using a script, call the script from BIMP and get the list of files as pngs without alpha. Unfortunately BIMP also makes a copy of the originals as well. 
Messy. Dead easy with ImageMagick or if a GUI is a requirement XnView.

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