Store graphics are easy to outsource badly, because every portfolio looks polished at full size on a desktop monitor. What you're actually buying is speed, consistency and files that survive the trip into a theme section.
Look at their graphics live, on a phone. A banner that looks sharp in a portfolio can lose its headline to a mobile crop or shrink its price to unreadable. Ask for two or three live Shopify stores they produce graphics for and open them on your phone before the call.
Test turnaround with one paid asset. This work is recurring, so the working relationship matters more than any single piece. Commission one sale banner at the quoted rate, note how long it takes and how many rounds it needs. That tells you more than a portfolio does.
Ask for editable source files and templates. The deliverable should include layered source files and, for recurring work, templates your team can update for the next sale. A designer who only hands over flattened JPGs has made you dependent on them for every date change.
Check they design to your brand, not their taste. An ecommerce graphic designer should ask for your fonts, colour codes and any guidelines before the first draft. If they don't, expect graphics that look fine one at a time and like five different brands on the homepage.
Red flags. Portfolios built from stock templates with no ecommerce work. Quotes that don't say how many revisions are included. No questions about which theme you use or where the image will sit. And a default habit of putting every word of copy inside the image.