There is no possible reason why you would need metadata for any kind of thumbnail, much less a user icon.
In any event, tumblr already resizes all large images to a maximum of 1280 pixels on either axis. If you were very concerned with maintaining images in their original state, this would be driving you up a wall. Good thing it's trivially easy to use alternative image hosting sites, like flickr or photobucket, on tumblr.
There's metadata and there's metadata. Sure, EXIF, IPTC and XMP data is usually entirely useless for web images. But colour profile information actually has the information needed to correctly render the colours in the image.
You could argue that small thumbnails don't matter. I would agree for small sizes, but as the size increases, it becomes important to preserve the ICC profile data. With bandwidth having become cheap, and processing power having become abundant, these days it's often appropriate to downscale images in the browser; so instead of storing thumbnail as 40x40, 60x60, 100x100 etc. depending on the use cases, just store a flexible version (say, 150x150) and let the browser scale. At that resolution, I would keep the ICC data.
Well, except that this particular color-profile was for the sRGB color space, which is exactly what every browser and image-viewer on the planet defaults to if no color-profile is supplied. It'd be like shipping an ASCII text file with a Latin1-to-Unicode mapping table.
Yep. Safari (since 2.0), Chrome (since 15, I believe), Firefox (supported since 3.0, but only enabled in 3.5 and later) and IE (since 9.0) all use embedded ICC metadata. No idea about other browsers.
In any event, tumblr already resizes all large images to a maximum of 1280 pixels on either axis. If you were very concerned with maintaining images in their original state, this would be driving you up a wall. Good thing it's trivially easy to use alternative image hosting sites, like flickr or photobucket, on tumblr.