It's possible.
Example for firmware 1.0.9.30 :
Download dashUI v88/v83 [UPDATE ONE]-[NEW ICON AND ANIMATION] | Xiaomi Mi Band 4 | GeekDoing
Decompil with : MB4 Resource pack/unpack | Xiaomi Mi Band 4 | GeekDoing
Download snapchat picture and resize 36x36 with "paint", save as "png" + TinyPNG...