What's the simplest and best free Paint or Draw software

I have an iMac with 2.8 GHz Intel Core 2 Duo processor running OS 10.10.3.

I want to draw/paint (?) lines for walking routes over the UK Ordnance Survey maps I can capture via BingMaps - then attach results to mails to my walking friends, so they know where we will walk in advance.

I need software suitable to draw over the OS footpaths, without totally obliterating them - probably bright and bold but translucent dotted lines ?

I want to add text.

Working in layers might be useful.

The results must retain the fine detail of BingMaps OS maps after printing.

Any suggestions ?

casablanca24