The article provided in the link (with your question) fails to define how the theme of love is used or expressed in Imagism. What the article does do is provide a brief overview of the movement. Essentially, the Imagist is exact (in language and word choice), refuses to conform in poetic form (use free verse), are unrestricted in regards to matter, and true to image (no figuratives, only literals).
Therefore, the theme of love would be a hard theme for the typical Imagist. Given that love is an idea (or an abstract), an Imagist would not be able to describe it for all readers concretely. Essentially, the Imagist would construct for readers his or her own idea of what love in is a concrete way.
In regards to the link provided, love (or the theme of love) would be described using very concrete language. Lack of concrete language would not support the Imagist's ideology. Given the unrestricted nature of subject matter, different types of love would not be taboo (for the true Imagist). The hardest part of love in the Imagist text would be the use of how love exists in image form. Given that all people define love differently, this would be the most difficult for the Imagist.
Therefore, the theme of love, in the Imagist era, would be used to define a very specific relationship or ideal that all readers could recognize universally.