If you're referencing the Shopify speed tool, a score of 35 is good if you're using multiple store facing apps. The score is heavily based on 3rd party apps, and not actual speed (because it's based on Google Lighthouse, the same tool that powers the general performance analysis tool Google PageSpeedInsights).
That said, it's always good to check to see if you can improve your performance without sacrificing apps.
To check if there are optimization issues on your site:
1) Run your site / pages through the Shopify Analyzer. That's a free tool my team built for the community, the first performance analysis tool built specifically for Shopify sites, and with metrics based on over 1,000 Shopify sites we've analyzed.
2) Check your them files to see if there is any leftover code from apps you've deleted. When you delete an app, it loses access to your store, so cannot clean up after itself. We see this all the time, old app code or scripts loading unnecessarily. The main spot is usually theme.liquid, but I'd also check your product, cart, and collection templates. Make sure you backup your theme and work on that backup theme, so you don't accidentally affect the live site if you make a mistake. If you're not familiar editing themes, this guide will help you: https://speedboostr.com/how-to-safely-edit-your-shopify-theme.
3) To identify and fix coding errors, the console is a good place to start: https://speedboostr.com/finding-and-fixing-console-errors.
To your question coding errors vs picture size it depends on the severity of the coding error. Sometimes they're harmless, sometimes they cause major bottlenecks. You'll want to analyze, debug, and test to figure that out, but I would address both of them if you have the time.
If you prefer to have professionals handle this for you, send a message to my team at speedboostr.com/contact. We do a free analysis, so you can start there and see what the recommendations are from our site analyzer, then decide if it makes sense to work on a project.