Difficult if you don’t know the original format, but XnView click here does read Mac Paint files for example, along with a few others (~_~)
It is free, and often recommended, so nothing much to lose by downloading it, except time.
OnePlus 5 review
See the work of famous artists playing with toys
iPad Pro 10.5in (2017) review
Comment faire une capture d’écran sur un Mac ?