transparent png uploaded in sap doesn't work in LCD (from MIME it's ok)

Designer only supports transparency on 8 bit images.
Make sure your png's are of type PNG-8.

Yes, they are. The problem is in following - same picture (8bit, png) is put in MIME, and in SAP(as default image, on Production) and in form if i get xstring for picture in MIME that picture has preserved it's transparency, but when i get xstring for picture in SAP picture has white background:(. I also have a function that only displays picture and then i can see that picture is properly uploaded in SAP (you can see very well the diference between white and transparent backgound). Only conclusion I have is that is something wrong with the way i'm getting xstring for picture from SAP, it loses transparency somehow..

Sorry,

I cannot help you with xstring.
You should check the documentation if transparency is supported.