Storyline 3 : How to convert PNG files to JPEG

I am having a similar problem to some users above -- using Storyline 360, latest update, Modern Player/Output to HTML5 -- all the jpgs get output as much larger pngs in the output /mobile directory, introducing delays while the course runs for the users.

I import the jpg files directly into Storyline, from a subdirectory of the job folder on my hard drive, at their existing size, and scale/crop them in the program. (love this feature!)

In some cases I put a Character (png) over a background (jpg); in the /mobile directory, these images are exported separately, both as pngs.

I have used tinypng.com as recommended by another user as a workaround, to shrink the output files, but would much prefer a method to keep jpgs as jpgs.

Attached is the first page of the project.

(9/8 update:  I submitted this as a Case to the Support Team, so will update with solution.)