Thank you for your feedback.
I know the originals are bmp, but I couldn't upload the image here as bmp, so I had to convert it in png. I guess converting it back shouldn't be a problem.
Yes, some of the icons were very problematic with their original tango design because of the lack of pixels, so I chose to change the design to keep them legible. Do I have to change them back and try to keep them as close to the originals as possible (even if it's ugly) ?
The icons may indeed be slightly bigger, but I didn't experience display problems yet.
By the way, is there a high-res version of the icons ? So far I found only already sized down versions. It would help me to improve the result.