If the photographer shoots in Adobe RGB, the pictures will have a greater dynamic range than if they were shot in sRGB. This greater dynamic range is visible on your computer, but the web can only display things in sRGB. The standard webkit used by web browsers is just not able to display all of the color that is recorded in an Adobe RGB image.
Whatever software the photographer is using is likely embedding the Adobe RGB color space into the file when saving / exporting the file. Then, Picassa on your computer is converting them to the "sRGB IEC61966-2.1" colorspace when it imports them, but even if it didn't they would look desaturated once uploaded to web anyway.
So, if you convert the colorspace to sRGB before uploading them, you may be able to retain some of the color in the original images. One easy way to do this would be to install The GIMP or some other editing software on your computer (I mention The GIMP because I know it can do this but other software might be just as good), in The GIMP's preferences set the working colorspace to "sRGB IEC61966-2.1", then open any image and The GIMP should prompt you to change the ICC Color Profile, it will display the "Current Color Profile" and then offer a pull-down menu from which you can choose "sRGB IEC61966-2.1", save the file and you should have a nice looking jpg that won't desaturate when uploaded (or not as much at least).
I've used this process before with images that had been shot in Adobe RGB and found that The GIMP does a good job of creating an sRGB file that doesn't look too desaturated (at least compared to the desaturation you get when just uploading the Adobe RGB version).
Also, you could ask the photographers to save the files they submit to you with "sRGB" as the embedded colorspace to begin with. Then you won't have to do any of the above work. Some of them might not know how to do that or if it even can be done with the editing software they use. But, couldn't hurt to ask them anyway.
If it's Mac users only that have this issue, they may be using Aperture as it's an Apple product that doesn't have a Windows version. There is a way to set the color profile when exporting to jpg in Aperture, but it may not be obvious to many users.
In Aperture when you go to export a "Version", the dialog that comes up has a pull-down menu that usually defaults to "jpeg - Original Size" but you can click on it and choose from a bunch of other sizes. At the bottom of that pull-down menu is the option "Edit..." which if selected, brings up a dialog that has a bunch more options including the colorspace. You can select "sRGB IEC61966-2.1" and then click "OK" and then "Export Versions".