KuickShow displays images using all available space, without wasting it with bars or menus. It also has smart automatic zooming which scales images according to size. You can use the integrated file browser to choose images, or navigate through a folder with the mousewheel or the keyboard. It also has fullscreen view and slideshow, and supports a wide range of image formats.
MRML for KDE is a KDE MRML client which offers integration of content-based queries into Konqueror/KDE. It consists of a KPart (which could be embedded into Konqueror for example) and a kio-slave for communication with the MRML server. You can right-click on an image and choose "Search for similar images..." in the context-menu, which will perform a query on a server and present you a thumbnail view of the results. You can also refine queries by giving relevance feedback.