<?php
// i will use the global variable $wpdb
global $wpdb;
// i get all the terms from taxonomy 'solucoes' with the ID from the loop
$solucoes = wp_get_object_terms(get_the_ID(), 'solucoes');
// Loop
foreach($solucoes as $solucao) :
// I get the thumbnail_id from the term
$solucaoThumbnailId = $wpdb->get_results('SELECT meta_value FROM wp_postmeta WHERE post_id = "' . $solucao->term_id . '" AND meta_key = "_thumbnail_id"');
// If my get is TRUE
if($solucaoThumbnailId) {
// I get the relative URL from the upload inside the wp_postmeta table
$solucaoThumbnail = $wpdb->get_results('SELECT meta_value FROM wp_postmeta WHERE post_id = "' . $solucaoThumbnailId[0]->meta_value . '"');
// And use this. :)
echo '<li><a title="' . $solucao->name . '"><img src="' . get_template_directory_uri() . '../../../uploads/' . $solucaoThumbnail[0]->meta_value . '" width="50" height="50" /></a></li>';
}
// End loop
endforeach;
?>