Here is a clean Navkar Mantra you can directly use as an image (for wallpaper/poster etc.). You can save this text on a plain background in any photo editor / Canva to make your own image:

Devanagari (common form) नमो अरिहंताणं। नमो सिद्धाणं। नमो आयरियाणं। नमो उवज्जायाणं। नमो लोए सव्वसाहूणं॥

एसो पंच णमोक्कारो, सव्व पावप्पणासणो। मंगलाणं च सव्वेसिं, पढमं हवइ मंगलं॥

If you prefer Latin script for the image:

Namo Arihantāṇam Namo Siddhāṇam Namo Āyariyāṇam Namo Uvajjhāyāṇam Namo Loe Savva Sāhūṇam

Eso pañcha Namokkāro, Savva Pāvappaṇāsaṇo Maṅgalāṇaṃ ca Savvesiṃ, Paḍhamaṃ Havai Maṅgalaṃ

You can read more about Navkar Mantra and its meaning here