-- Traits --
TRY is a Plant Trait Database. More formally, it's a network of vegetation scientists headed by DIVERSITAS/IGBP, and the Max Planck Institute for Biogeochemistry, providing a global archive of curated plant traits.
This is an online resource for empirical data on how biological traits respond to environmental drivers such as temperature, light, and salinity.
Nate and his collaborators have compiled a ton of traits for species all over the world (though focused on the tropics).
Coyle et al. 2014
Appendix 2 of their 2014 Ecography paper lists primary literature sources and publicly available trait databases they used to examine 3 traits: leaf nitrogen, seed mass, and wood density.
-- Images --
An international Web database that contains thousands of high-resolution photographs and other images of plant and animal specimens.
Botanical Images Databases
This site provides dozens of links to databases containing plant images. There are also regional databases provided (e.g., Pflanzenphotos.de).
This site contains both text sources (e.g., lectures and essays) as well as links to morphology picture databases (e.g., Botanical Image Database of the University of Basel).
See also FAO Online Catalogues for a number of morphological sources in both English and German.