Hola Iván,
When you set the widget to use the “stock” thumbnails, WPP calls the get_the_post_thumbnail() function to retrieve the thumbnail image using the specified size.
This is what WordPress is returning in your case:
<img width="2048" height="1367" src="https://tbv.wancora.cat/wp-content/uploads/2018/03/10535679_308270449334193_1014144952197942695_o.jpg" class="attachment-post-thumbnail size-post-thumbnail wp-post-image" alt="" srcset="https://tbv.wancora.cat/wp-content/uploads/2018/03/10535679_308270449334193_1014144952197942695_o.jpg 2048w, https://tbv.wancora.cat/wp-content/uploads/2018/03/10535679_308270449334193_1014144952197942695_o-300x200.jpg 300w, https://tbv.wancora.cat/wp-content/uploads/2018/03/10535679_308270449334193_1014144952197942695_o-768x513.jpg 768w, https://tbv.wancora.cat/wp-content/uploads/2018/03/10535679_308270449334193_1014144952197942695_o-1030x688.jpg 1030w, https://tbv.wancora.cat/wp-content/uploads/2018/03/10535679_308270449334193_1014144952197942695_o-1500x1001.jpg 1500w, https://tbv.wancora.cat/wp-content/uploads/2018/03/10535679_308270449334193_1014144952197942695_o-705x471.jpg 705w, https://tbv.wancora.cat/wp-content/uploads/2018/03/10535679_308270449334193_1014144952197942695_o-450x300.jpg 450w" sizes="(max-width: 2048px) 100vw, 2048px">
It seems WordPress failed to find a thumbnail with size 710×375 in your Uploads folder so it returned the original image instead.
Try using Alex Mill’s Regenerate Thumbnails plugin to (re)generate the missing image size(s) so WPP can display the right one.