The only reason you are lost is that you do not understand which workflow to use for the task at hand which is sculpting \ detailing a model for 3D printing.
Do not use the Paint Room for your specific task.
Import the STL model into the voxel room either as a voxel object or surface model object.
Use your material image or a mask image to create your real depth in the model surface.
Export the model as a STL model file. The model will be exported with the real depth created by polygons.
ShapeWays has a limit on the amount of polygons than can be 3D printed which is one million. I do not know the polygon limit for other 3D printing websites.
When exporting you can reduce the amount of polygons or in other words polygon reduction through a smart decimation routine which keeps the details as much as possible.
It is not in the scope of this answer to tell you in detail how to do the above but these are the correct steps. Also I am not including information about preparing your model for 3D printing.
Sorry to port you off to tutorial land again for this...
3DCoat official YouTube Channel.
For 3D printing.
The model in the image is 950,000 polygons. I used the low quality image as the material image file from your post to create the depth.
Not shown for quality.
Viewed in 3DC and viewing the STL model as real polygons in Blender's viewport.