"Missing with rel="self"": because you have many result and pipe uses pagination, the <atom:link> links to the next page of your results.
However, you should really look at the others 2 problems (well, the first at least):
"guid values must not be duplicated within a feed": you probably have duplicates in your feed and that's not nice at all for the viewer. Either use the Unique module to filter out duplicates, or revise your guid scheme.
"Missing enclosure attribute: length": Check that article. Basically, the enclosure field should have the attributes type, length, and url. The length attribute specifies the length of the media file. Obviously not really applicable in case of images, the enclosure is still the most portable implantation: either ignore the warning or put a length=0 value.