Our max photo size is 5 MB, which is high-resolution enough to be displayed on large computer monitors. There will be no trade-off on image quality if the image is resized within that range.
There are many free photo tools available on the internet to resize images! We recommend this one from Shopify and to resize all photos to the "large" setting. Also, you might be able to resize your images by simply opening them in your photo editor of choice and selecting “Image Resize” in the tool menu.