Reduce map size

I have a few issues with some of my players not having enough memory on their system to reliably run the maps I put in my campaign. I'm only blaming myself (I put in 4500x3136 pixel maps and populated them before I knew it would cause problems), but my question is this:

Is there a way to reduce file size of my maps AFTER I have already gone through and drawn in FoW, added markers for room descriptions, and populated the map with monsters and NPCs? I'm guessing that if I surreptitiously modify the jpg in the campaign folder and resample it with lower resolution, it would screw things up.

I just really hate to have to start over and lose all the hours I put in preparing.

 

Would it be possible to find the jpg, open it in Photoshop, then save it with a reduced quality setting while keeping the pixel count the same? That seems like it would gain me a bit of reduction, but would it mess up d20Pro?

Edit:
I tried deleting the "MapFloor" layer in drawing tools and replacing it with a version that was the same pixel size but reduced quality and I couldn't manage to get the grids lined up. All of the tweaking I had done when I created the map wasn't possible through this method.

Also, I think my main problem is that I was a dufus and told D20Pro to save my maps as a PNG, which makes them ginormous.