The postcard shows another still life painting by Greg Bolaños entitled Watermelon. I couldn't find additional information about the painting nor its location (private collection or museum), but National Bookstore owner Socorro Ramos has still-life collections done by Bolaños.
Watermelon refers to both fruit and plant of a vine-like (climber and trailer) herb, and one of the most common types of melon. The watermelon fruit, loosely considered a type of melon, has a smooth exterior rind (green and yellow) and a juicy, sweet, usually pink, red, or yellow, but sometimes orange, interior flesh.
Watermelon Wikipedia Entry