It's easy to add your MapKnitter exported map as a background layer when editing OpenStreetMap in Potlatch 2. In this example I'm using a map of Mestia, a city in the Svaneti region of Georgia.

New technique

With help from Andreas Trawoeger from the OSM-talk list, we set up a "deep link" so that simply by clicking Use your map to contribute to OpenStreetMap underneath the Export formats area, you can open your map directly in OpenStreetMap's Potlatch 2 editor by clicking "Use your map to contribute to OpenStreetMap":

Note: this only works for maps which have been released under open licenses, such as CC-BY or public domain.

Old technique (no longer necessary)

First, open the "backgrounds" menu and choose "Add":

Then, add a new background layer, using your map's "OSM-style TMS" link, with added z,x,y like this:

https://mapknitter.org/tms/mestia/alt/$z/$x/$y.png

Where "mestia" is your map's name. See screenshot:

You can turn off "dimming" to see the map more clearly:

Zoom in and see how well your balloon map fits existing data. Then trace to add more features: