A lot can be gone with images using Gimp;but gimp uses an additive
process rather than the subtractive most photographers use for making
their film based color prints.If you have any color darkroom film
based background using gimp is like learning to see backwards.Their is
nothing wrong with using the additive process but it is a bit of a
learning curve,
Picasa on the other hand uses processes that are more familiar.
It has some very good tweaking tools.I currently shoot film;both
transparency and negative;but process prints and images for the web
digitally.I use gimp for most of my processing and then use picassa to
pour on the shine.
The one thing I do not like about picassa is that it only saves images
as jpegs.