But you can mention negative imagery if you tie it to the opposition. Thus you don't say "we aren't the party of throat-cutters". You say "The Republicans showcase violence like throat cutting at their convention". It's not high road versus low road; it's calling a scoundrel a scoundrel, "reality therapy". Too much high road, pure and clean looks weak and meek.
It's the same way Republicans mention "taxes". They call themselves the "tax relievers" and call us the "tax-and-spenders". The antidote is call them the "borrow-and-spend" party, and call us the "job creators", the "builders of infrastructure", the "educators for our future", etc.
Turn their phrasing around. So where they call themselves "tax cutters", say the "Republicans will cut the middle class to shreds" or "in ribbons" or "to pieces". That way when voters start to hear "cut" they start to feel they are being cut by Republicans. Say things like "The Republicans would make the middle class die by a thousand cuts."
"War on women" is a successful meme, especially when it is tied to the party as in "Republican war on women". It's negative, it's not high-roading, and it's not meek to bring the battle to the Republicans by making voters associate belligerent, violent, destructive imagery with them. They've been doing it to the Democratic Party for decades, because it is effective. We need to be even more effective.
You are right about terms like "gun-justice". That frames the debate as "guns = justice", which is nuts of course. So, taking the cue from Lakoff, talk about "relief from gun violence" and "peace and safety".
The modern world is more visual than ever. So bring it to the Republicans visually, audibly, and verbally by framing our policies and successes with positives visually, audibly, and verbally and framing their policies and failures with negatives visually, audibly, and verbally.