The top 2 places to spend your money when designing an ecommerce store are
Using the right image, in the right size and format, is important in giving off the right impression.
I recommend using Canva for your image editing and graphic design. They have a great free option that will do most of the work you need.
Below is a simple chart to help you with sizing your images correctly for your website.
These are just guidelines and can be adjusted as you feel necessary. Use these recommendations as a starting point in sizing your images and graphics.
If you have been following along via my Youtube channel with the basic store setup you will find several images below the chart that you can use to follow along.
Use these demo images to insert into your store as you learn to work within Shopify. Most of the images also have the dimensions, in pixels, in the lower corner to help you remember.