I don’t know of a way to alter the PNGs to make them be JPEG, while still in the model. I do know a way to get the same end result.
- Take your model and export it as Collada. In the Collada export Options…, select these ones:
You get a .DAE file and a folder of textures. Open the .DAE in a text editor, and find everywhere that .png is. Edit the text to make those be .jpg. Save the file.
In the folder of textures will be some number of files. Open any that are PNG in an image editor. On Windows, the Paint program does nicely. Use the File, Save As option, to save a JPG with the same name, into the textures folder (next to where the PNG is).
In a new SketchUp file, use the File/Import option, and either set it to All Supported Types, or specifically Collada.
For the Collada options, make sure both boxes are checked:
As you do the import, click at the Origin to place the imported geometry, so that it ends up in the same place it was before.
You should now have a model that looks the same as before, but will be a lot smaller file.
If in your original model you had a lot of scenes, you could save a copy of that file, delete all of the objects, and purge unused, so that you are only left with the scenes and the styles. Now copy everything in the imported Collada document, and do an Edit/Paste in Place, in the empty copy that you made.
One difficulty you will hit on Windows is that when you make a new document it will expect you to close the existing one, and if you are jumping between files to copy and paste things, that can be inconvenient. You can get around that by shift-clicking on the SketchUp icon in the Taskbar. That will give you a second or third copy of SketchUp running, and then you can keep two or three documents open at the same time.