Okay, here is a workaround I found if anyone is looking to do this in the future...
I do have a tool, DungeonDraft, that allows me to export a Universal Battle Map/dd2vtt file.
Step 1: Export universal battlemap example: ExampleMap.dd2vtt
Step 2: Download this tool from Github: https://github.com/beamer159/uvtt-insect/releases/tag/v0.1.0 (you only need the .exe file)
Step 3: Place the uvtt-insect.exe file in the same folder as your exported battlemap
Step 4: In FileExplorer, look for the File tab at the top right; click on it; select Windows Powershell and run it as administrator
Step 5: place the following command into the Powershell window:
.\uvtt-insect extract -i ExampleMap.dd2vtt
Result: a new PNG file named ExampleMap.png will show up in the folder.
Step 6: Open the PNG with paint.net and edit to your heart's content.
Step 7: Save the file , does NOT need to be PNG, can be WEBP (recommended), not sure what other file types will work (same folder location)
Step 8: place the following command into the Powershell window:
.\uvtt-insect insert -i ExampleMap.dd2vtt -m ExampleMap.webp
Result: the dd2vtt file contains the updated image.