On a Mac, you can substitute a forward slash "/" for a colon within a "Save As:" dialog, or when editing a filename in the Finder. Or just type "\:" in the Terminal. This way, you can create and upload notes with colons in their names.
Uploading images from a Mac to the iPad is fiddly, but possible.
Here's what worked for me USING TRUNKSYNC.PY. Can somebody (with a backup) check this for Dropbox sync?
1) Copy the image, say testimage.png, to the directory where images are stored locally (see trunksync docs)
2) Create a new note called "File:testimage.png.txt" in the directory where notes are stored locally
3) Edit the note to display "Title: File:testimage.png" on the first and only line
Note that trunksync.py typically initializes the header fields of notes created locally. However, trunksync.py uses a slightly different convention and appends ".txt" to the end of the Title field above. This, unfortunately, disables the image display within the note.
Actually, with trunksync.py at least, the note's filename can be anything "BlahBlahBlah.txt", as long as the Title field is included. I don't recommend this, because you will have to remember two names for the same logical note: the local filename, and the iPad name given in the Title field. Another danger is overwriting notes after copying files locally, e.g. BlahBlahBlah2.txt, if you forget to change the Title header field as well.