Hi @Meredithx !
Trevor from SHOPLAZZA here. Welcome to the Community!
Regarding the size of the picture, the picture can be cropped according to the display effect on the website. Some themes cannot be adapted. You need to upload the pictures on the mobile phone and PC separately. The following picture size data is for your reference:
Banner: PC: 1920x860 Mobile: 750x800
Main image: 1000x1000
I hope this is helpful, please let me know if it works for you, thanks!
If you have any further questions, be sure to reach out to SHOPLAZZA support.