What I get from trying to use /img is a 403 error, which means the server cannot access the file.
Unfortunately, this seems to be a Dropbox issue, I recommend using something like https://catbox.moe/ instead.
In more recent versions of MCGalaxy, you can use links from dropbox and the like as long as it is less than 128 characters and has the file extension in it.
As for the MCGalaxy.exe file, that would be on the computer of whoever's hosting the server.
extra/images folder is relative to the MCGalaxy.exe file, not your C drive.
You need to place your image file here, then run /imageprint 26
(without the file extension)
bots
extra
----images
--------26.bmp
levels
MCGalaxy.exe
MCGalaxy_.dll
etc