zoom into the model and make it full screen then export as png file.

Failing that - load it into your slicer, zoom in and use a screen grab.

Another approach would be to export the model in a format that can be imported into blender or photoshop, with full 3d information. And export as jpg from there.
Surely one of the formats openscad will export must be importable by a hybrid image/modelling program.

Only thing I've got installed on this machine is gimp 2.8 - and that's not a modelling package.
Pretty sure blender should do the job.