Thanks for reaching out. I’ll need a little more information so that I can help you with this issue.
Could you please share the version of Lightroom & the operating system you are working with? What is the format of the images you work on? Are they JPEG or RAW?
Has it worked as expected in the past? If yes, try to compare the dimension (size) of images that worked and the newly exported images that don't work as expected.
Could you please share sample images from the past and the new exports? Do the images appear the same in the system Photo viewer? Different web platforms have their method of compression and handling user images.
It is advised to contact Prestashop support to understand the ideal sizes/dimensions of images as per your needs.