Typically you would just go and lodge a purchase invoice set to the date you acquired the paintings and post them to a balance sheet code like "Stock". You can pay for these items from the Proprietor Drawings Account.
Regarding the differentiation between your personal and business accounts, there really isn't any. As a sole trader your business and personal accounts are one and the same. Accounting as a sole trader involves entering only those transactions that relate to your business (irrespective of the bank account) and leaving out everything else.
There's a useful guide here that will explain a few of the concepts
Sole Trader Bookkeeping - The Basics
Just to add I'm not an accountant, so it's always worth seeking professional advice. There may be capital gains involved here so if the amounts are significant it's highly advisable to seek a professional opinion.