While not a concrete answer...here is what I have that might shed some light on the issue.
1- I've been using Picasa (Mac and PC) since 2008 with my Canon SD1100 (point and shoot, JPG output). No issues with color on these shots.
2- I briefly tested Picasa on my Mac with my Canon T3i RAW files. I don't recall seeing any issue but I wasn't really looking for much there and was more surprised they even loaded.
3- Found this on a brief search of google forums
RAW formats are proprietary and vary with the camera manufacturer and model. New RAW formats are periodically added to new versions of Picasa, so it may just be a matter of time before import support is added for that particular format. There is no published schedule, so we don't know if or when this might happen. In any case, there will be no advantage to using the RAW format within Picasa, even it if does read them. Picasa is not a RAW processor and can export only JPGs
So, depending on the format of the file your S100 is producing, it is potentially just a limit of Picasa for the time being until the software is updated. Maybe try other formats if they are possible and see if the tint disappears.