All networks are different and follow different rules when it comes to images sizes. If you have posted to Pinterest with Blog2Social and Pinterest doesn't displays your image or not in a way you want, the size of your image could be the reason.
Overview of the images sizes suitable for Pinterest
Although Blog2Social adapts your blog's images to the respective networks you are posting them to, images that are too large or too small will result in, either not being posted at all or appearing pixeled as Blog2Social tries to fill the images frame in the network.
As a ground rule for Pinterest, your images should not exceed 10 MB. Different numbers of pixel are suitable for diffrent purposes.