HIVE

While the site converter is pretty convenient, BM does give more control over the result ^^
It also seems like the site converter expects a borderless image, which a lot of people dismiss/are not aware of, resulting in improper ATT/UPG/SSC borders with the BTN border still visible in them
Quality-wise, it does seem like work is being done in that direction, so hopefully at some point this won't be an issue ^^

The Icons can be uploaded in .blp format? But the site asks for .dds, if I don't upload a raw PNG: .dds is still good, or .blp is the finest?

You can upload BLP. I think DDS is still compressed into BLP after.

.blp files are distorted too by the Website. DDS format still holds it's quality after upload.

BLP are not being modified by the site, it is a direct file upload.

Here, I made a pure .blp upload (directly uploaded the same icons in .blp format and in .dds format)

I also forgot to mention it: I made the icons with .blp format by Button Manager, after that I upload the .blp files to the website. Also, .dds upload is made with the following process: Icons first made with Button Manager to .png format, and then the .png icons converted with GIMP to .dds, after that I upload the .dds icons to the site.

Must be something wrong on your end. (I am not sure how you are uploading the icons), maybe you lost some quality when converting your PNG to BLP in the first place, I can't say - from my eye - i see no difference between my PNG (originals) and BLP (converted), here is a comparison of the Original and double-converted image from PNG to BLP to PNG after.
Even if there is any quality decrease (which might happen because of the conversion itself from PNG to BLP), I can't notice it much.
(so maybe in such cases DDS might be a bit superior - I didn't test much the quality change from PNG to DDS)

Thanks for the info, so the problem is on my end. Is there any method to convert png/other formats to .blp format other than with Button Manager? I also reseize with Button Manager.