Somehow you can think that "I've always had a vivid imagination" is "eu sempre tenho tido uma imaginação fértil." in sense. That is, my imagination don´t have failed to be vivid. Whereas "I always had" would be "eu sempre tive..." As you see, it´s a slight and subtle difference.
Sometimes the effect is the same, or almost, so you could see it translated as "eu sempre tive" in Portuguese, but with that sense of "eu sempre tenho tido...". This is a fact that it´s not always explained, they translate certain expressions and sentences but the learner has to find the subtleties of them.