Only with permission of the band to do so - there is usually a licensing fee involved. You can contact their record company to find the proper agency to obtain the licensing rights.
It is legal, but if you do that expect the band to sue for you for all your profits. You must invent band names if you want to stay clear on that matter. Your art work is totally yours to do with as you please, but check copy-write laws to be sure before you go ahead. If you wish to do Band Art posters and tee-shirt for real on-going bands, then they and their manager must sign a contract with you, for the franchise rights - they will expect their cut.
Only if it is your band or if the band gives you premission to do so
